diff --git a/pkg/front_end/lib/src/fasta/kernel/constant_evaluator.dart b/pkg/front_end/lib/src/fasta/kernel/constant_evaluator.dart
index 7d3a1b06dee..bab812df1e2 100644
--- a/pkg/front_end/lib/src/fasta/kernel/constant_evaluator.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/constant_evaluator.dart
@@ -1666,7 +1666,7 @@ class ConstantsTransformer extends RemovingTransformer {
 
     List<Statement> replacementStatements = [
       ...matchingCache.declarations,
-      // TODO(cstefantsova): Figure out the right exception to throw.
+      // TODO(cstefantsova): Provide a better diagnostic message.
       createIfStatement(
           createNot(readMatchingExpression),
           createExpressionStatement(createThrow(createConstructorInvocation(
@@ -1726,12 +1726,15 @@ class ConstantsTransformer extends RemovingTransformer {
     List<Statement> replacementStatements = [
       ...node.pattern.declaredVariables,
       ...matchingCache.declarations,
-      // TODO(cstefantsova): Figure out the right exception to throw.
+      // TODO(cstefantsova): Provide a better diagnostic message.
       createIfStatement(
           createNot(readMatchingExpression),
           createExpressionStatement(createThrow(createConstructorInvocation(
-              typeEnvironment.coreTypes.reachabilityErrorConstructor,
-              createArguments([], fileOffset: node.fileOffset),
+              typeEnvironment.coreTypes.stateErrorConstructor,
+              createArguments([
+                createStringLiteral(messagePatternMatchingError.problemMessage,
+                    fileOffset: node.fileOffset)
+              ], fileOffset: node.fileOffset),
               fileOffset: node.fileOffset))),
           fileOffset: node.fileOffset),
     ];
diff --git a/pkg/front_end/testcases/dartdevc/issue51449.dart.strong.expect b/pkg/front_end/testcases/dartdevc/issue51449.dart.strong.expect
index 021d9f9ed81..b286fb19f0a 100644
--- a/pkg/front_end/testcases/dartdevc/issue51449.dart.strong.expect
+++ b/pkg/front_end/testcases/dartdevc/issue51449.dart.strong.expect
@@ -25,7 +25,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #0#0 = (2, "Hello");
     if(!(let final dynamic #t3 = #j#set(#0#0{(core::int, core::String)}.$1{core::int}){(core::int) → dynamic} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(#j#get(){() → core::int});
 }
diff --git a/pkg/front_end/testcases/dartdevc/issue51449.dart.strong.transformed.expect b/pkg/front_end/testcases/dartdevc/issue51449.dart.strong.transformed.expect
index ef7a5ea1503..b7f1ac47ede 100644
--- a/pkg/front_end/testcases/dartdevc/issue51449.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/dartdevc/issue51449.dart.strong.transformed.expect
@@ -25,7 +25,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #0#0 = (2, "Hello");
     if(!(let final dynamic #t3 = #j#set(#0#0{(core::int, core::String)}.$1{core::int}){(core::int) → dynamic} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(#j#get(){() → core::int});
 }
diff --git a/pkg/front_end/testcases/dartdevc/issue51449.dart.weak.expect b/pkg/front_end/testcases/dartdevc/issue51449.dart.weak.expect
index ac048cd908f..672c1f845dd 100644
--- a/pkg/front_end/testcases/dartdevc/issue51449.dart.weak.expect
+++ b/pkg/front_end/testcases/dartdevc/issue51449.dart.weak.expect
@@ -31,7 +31,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #0#0 = (2, "Hello");
     if(!(let final dynamic #t1 = #j#set(#0#0{(core::int, core::String)}.$1{core::int}){(core::int) → dynamic} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(#j#get(){() → core::int});
 }
diff --git a/pkg/front_end/testcases/dartdevc/issue51449.dart.weak.modular.expect b/pkg/front_end/testcases/dartdevc/issue51449.dart.weak.modular.expect
index ac048cd908f..672c1f845dd 100644
--- a/pkg/front_end/testcases/dartdevc/issue51449.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/dartdevc/issue51449.dart.weak.modular.expect
@@ -31,7 +31,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #0#0 = (2, "Hello");
     if(!(let final dynamic #t1 = #j#set(#0#0{(core::int, core::String)}.$1{core::int}){(core::int) → dynamic} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(#j#get(){() → core::int});
 }
diff --git a/pkg/front_end/testcases/dartdevc/issue51449.dart.weak.transformed.expect b/pkg/front_end/testcases/dartdevc/issue51449.dart.weak.transformed.expect
index 3d76a92c264..c2ef169c299 100644
--- a/pkg/front_end/testcases/dartdevc/issue51449.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/dartdevc/issue51449.dart.weak.transformed.expect
@@ -31,7 +31,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #0#0 = (2, "Hello");
     if(!(let final dynamic #t1 = #j#set(#0#0{(core::int, core::String)}.$1{core::int}){(core::int) → dynamic} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(#j#get(){() → core::int});
 }
diff --git a/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.strong.expect b/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.strong.expect
index 368ca906196..33c676f5cf7 100644
--- a/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.strong.expect
+++ b/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.strong.expect
@@ -30,7 +30,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   synthetic constructor •() → self::Class
@@ -46,7 +45,7 @@ static method errors(self::Class c) → dynamic {
   block {
     final synthesized dynamic #0#0 = 42;
     if(!true)
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   invalid-expression "pkg/front_end/testcases/extensions/explicit_invalid_access.dart:12:3: Error: Explicit extension application cannot be used as an expression.
   Extension(c) += 42;
diff --git a/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.strong.transformed.expect b/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.strong.transformed.expect
index d0639ab643f..a62c6e8dede 100644
--- a/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.strong.transformed.expect
@@ -30,7 +30,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   synthetic constructor •() → self::Class
@@ -46,7 +45,7 @@ static method errors(self::Class c) → dynamic {
   block {
     final synthesized dynamic #0#0 = 42;
     if(!true)
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   invalid-expression "pkg/front_end/testcases/extensions/explicit_invalid_access.dart:12:3: Error: Explicit extension application cannot be used as an expression.
   Extension(c) += 42;
diff --git a/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.weak.expect b/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.weak.expect
index 368ca906196..33c676f5cf7 100644
--- a/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.weak.expect
+++ b/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.weak.expect
@@ -30,7 +30,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   synthetic constructor •() → self::Class
@@ -46,7 +45,7 @@ static method errors(self::Class c) → dynamic {
   block {
     final synthesized dynamic #0#0 = 42;
     if(!true)
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   invalid-expression "pkg/front_end/testcases/extensions/explicit_invalid_access.dart:12:3: Error: Explicit extension application cannot be used as an expression.
   Extension(c) += 42;
diff --git a/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.weak.modular.expect b/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.weak.modular.expect
index 368ca906196..33c676f5cf7 100644
--- a/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.weak.modular.expect
@@ -30,7 +30,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   synthetic constructor •() → self::Class
@@ -46,7 +45,7 @@ static method errors(self::Class c) → dynamic {
   block {
     final synthesized dynamic #0#0 = 42;
     if(!true)
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   invalid-expression "pkg/front_end/testcases/extensions/explicit_invalid_access.dart:12:3: Error: Explicit extension application cannot be used as an expression.
   Extension(c) += 42;
diff --git a/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.weak.transformed.expect b/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.weak.transformed.expect
index d0639ab643f..a62c6e8dede 100644
--- a/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/extensions/explicit_invalid_access.dart.weak.transformed.expect
@@ -30,7 +30,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   synthetic constructor •() → self::Class
@@ -46,7 +45,7 @@ static method errors(self::Class c) → dynamic {
   block {
     final synthesized dynamic #0#0 = 42;
     if(!true)
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   invalid-expression "pkg/front_end/testcases/extensions/explicit_invalid_access.dart:12:3: Error: Explicit extension application cannot be used as an expression.
   Extension(c) += 42;
diff --git a/pkg/front_end/testcases/general/issue40242.dart.strong.expect b/pkg/front_end/testcases/general/issue40242.dart.strong.expect
index 82a824b25c8..c68d53ebcaa 100644
--- a/pkg/front_end/testcases/general/issue40242.dart.strong.expect
+++ b/pkg/front_end/testcases/general/issue40242.dart.strong.expect
@@ -20,7 +20,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class C extends core::Object {
   synthetic constructor •() → self::C
@@ -50,7 +49,7 @@ static method errors() → dynamic {
   block {
     final synthesized dynamic #0#0 = new self::C::•();
     if(!(let final dynamic #t1 = c2 = #0#0{self::C} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   invalid-expression "pkg/front_end/testcases/general/issue40242.dart:20:3: Error: Can't assign to the const variable 'c3'.
   c3 = null;
diff --git a/pkg/front_end/testcases/general/issue40242.dart.strong.transformed.expect b/pkg/front_end/testcases/general/issue40242.dart.strong.transformed.expect
index a6769b94613..17eacd360c5 100644
--- a/pkg/front_end/testcases/general/issue40242.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/general/issue40242.dart.strong.transformed.expect
@@ -20,7 +20,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class C extends core::Object {
   synthetic constructor •() → self::C
@@ -50,7 +49,7 @@ static method errors() → dynamic {
   block {
     final synthesized dynamic #0#0 = new self::C::•();
     if(!(let final self::C #t1 = c2 = #0#0{self::C} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   invalid-expression "pkg/front_end/testcases/general/issue40242.dart:20:3: Error: Can't assign to the const variable 'c3'.
   c3 = null;
diff --git a/pkg/front_end/testcases/general/issue40242.dart.weak.expect b/pkg/front_end/testcases/general/issue40242.dart.weak.expect
index 82a824b25c8..c68d53ebcaa 100644
--- a/pkg/front_end/testcases/general/issue40242.dart.weak.expect
+++ b/pkg/front_end/testcases/general/issue40242.dart.weak.expect
@@ -20,7 +20,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class C extends core::Object {
   synthetic constructor •() → self::C
@@ -50,7 +49,7 @@ static method errors() → dynamic {
   block {
     final synthesized dynamic #0#0 = new self::C::•();
     if(!(let final dynamic #t1 = c2 = #0#0{self::C} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   invalid-expression "pkg/front_end/testcases/general/issue40242.dart:20:3: Error: Can't assign to the const variable 'c3'.
   c3 = null;
diff --git a/pkg/front_end/testcases/general/issue40242.dart.weak.modular.expect b/pkg/front_end/testcases/general/issue40242.dart.weak.modular.expect
index 82a824b25c8..c68d53ebcaa 100644
--- a/pkg/front_end/testcases/general/issue40242.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/general/issue40242.dart.weak.modular.expect
@@ -20,7 +20,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class C extends core::Object {
   synthetic constructor •() → self::C
@@ -50,7 +49,7 @@ static method errors() → dynamic {
   block {
     final synthesized dynamic #0#0 = new self::C::•();
     if(!(let final dynamic #t1 = c2 = #0#0{self::C} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   invalid-expression "pkg/front_end/testcases/general/issue40242.dart:20:3: Error: Can't assign to the const variable 'c3'.
   c3 = null;
diff --git a/pkg/front_end/testcases/general/issue40242.dart.weak.transformed.expect b/pkg/front_end/testcases/general/issue40242.dart.weak.transformed.expect
index a6769b94613..17eacd360c5 100644
--- a/pkg/front_end/testcases/general/issue40242.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/general/issue40242.dart.weak.transformed.expect
@@ -20,7 +20,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class C extends core::Object {
   synthetic constructor •() → self::C
@@ -50,7 +49,7 @@ static method errors() → dynamic {
   block {
     final synthesized dynamic #0#0 = new self::C::•();
     if(!(let final self::C #t1 = c2 = #0#0{self::C} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   invalid-expression "pkg/front_end/testcases/general/issue40242.dart:20:3: Error: Can't assign to the const variable 'c3'.
   c3 = null;
diff --git a/pkg/front_end/testcases/late_lowering/issue51449.dart.strong.expect b/pkg/front_end/testcases/late_lowering/issue51449.dart.strong.expect
index 021d9f9ed81..b286fb19f0a 100644
--- a/pkg/front_end/testcases/late_lowering/issue51449.dart.strong.expect
+++ b/pkg/front_end/testcases/late_lowering/issue51449.dart.strong.expect
@@ -25,7 +25,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #0#0 = (2, "Hello");
     if(!(let final dynamic #t3 = #j#set(#0#0{(core::int, core::String)}.$1{core::int}){(core::int) → dynamic} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(#j#get(){() → core::int});
 }
diff --git a/pkg/front_end/testcases/late_lowering/issue51449.dart.strong.transformed.expect b/pkg/front_end/testcases/late_lowering/issue51449.dart.strong.transformed.expect
index f9fe98d291c..06232d1d42b 100644
--- a/pkg/front_end/testcases/late_lowering/issue51449.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/late_lowering/issue51449.dart.strong.transformed.expect
@@ -25,7 +25,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #0#0 = (2, "Hello");
     if(!(let final dynamic #t3 = #j#set(#0#0{(core::int, core::String)}.$1{core::int}){(core::int) → dynamic} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(#j#get(){() → core::int});
 }
diff --git a/pkg/front_end/testcases/late_lowering/issue51449.dart.weak.expect b/pkg/front_end/testcases/late_lowering/issue51449.dart.weak.expect
index ac048cd908f..672c1f845dd 100644
--- a/pkg/front_end/testcases/late_lowering/issue51449.dart.weak.expect
+++ b/pkg/front_end/testcases/late_lowering/issue51449.dart.weak.expect
@@ -31,7 +31,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #0#0 = (2, "Hello");
     if(!(let final dynamic #t1 = #j#set(#0#0{(core::int, core::String)}.$1{core::int}){(core::int) → dynamic} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(#j#get(){() → core::int});
 }
diff --git a/pkg/front_end/testcases/late_lowering/issue51449.dart.weak.modular.expect b/pkg/front_end/testcases/late_lowering/issue51449.dart.weak.modular.expect
index ac048cd908f..672c1f845dd 100644
--- a/pkg/front_end/testcases/late_lowering/issue51449.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/late_lowering/issue51449.dart.weak.modular.expect
@@ -31,7 +31,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #0#0 = (2, "Hello");
     if(!(let final dynamic #t1 = #j#set(#0#0{(core::int, core::String)}.$1{core::int}){(core::int) → dynamic} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(#j#get(){() → core::int});
 }
diff --git a/pkg/front_end/testcases/late_lowering/issue51449.dart.weak.transformed.expect b/pkg/front_end/testcases/late_lowering/issue51449.dart.weak.transformed.expect
index c685bd3971a..41c131958c6 100644
--- a/pkg/front_end/testcases/late_lowering/issue51449.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/late_lowering/issue51449.dart.weak.transformed.expect
@@ -31,7 +31,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #0#0 = (2, "Hello");
     if(!(let final dynamic #t1 = #j#set(#0#0{(core::int, core::String)}.$1{core::int}){(core::int) → dynamic} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(#j#get(){() → core::int});
 }
diff --git a/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.strong.expect b/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.strong.expect
index 8ebe1e623ad..7fa5bd26b55 100644
--- a/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.strong.expect
+++ b/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.strong.expect
@@ -81,14 +81,14 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
    ^^^^^^^^^^^^^^^^" && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:32:7: Error: Can't assign to the final variable '#initializedLateFinal'.
       initializedLateFinal, // Error
       ^^^^^^^^^^^^^^^^^^^^^" && (let final dynamic #t5 = definitelyUnassignedFinal = #0#0{(core::Object?, core::Object?, core::Object?)}.$3{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t6 = #definitelyUnassignedLateFinal#set(#1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int?){(core::int) → dynamic} in true) && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:35:5: Error: Final variable 'definitelyAssignedFinal' might already be assigned at this point.
     definitelyAssignedFinal] = y; // Error
     ^^^^^^^^^^^^^^^^^^^^^^^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -109,7 +109,7 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#3#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#3#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#9#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:37:17: Error: Final variable 'notDefinitelyAssignedFinal' might already be assigned at this point.
       field2: [[notDefinitelyAssignedFinal, _], // Error
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#16#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#16#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t11 = #notDefinitelyAssignedLateFinal#set((#2#16#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::[]}(1){(core::int) → dynamic} as{ForNonNullableByDefault} core::int?){(core::int) → dynamic} in true)))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.strong.transformed.expect b/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.strong.transformed.expect
index a52454072c9..7a6aff1bf5d 100644
--- a/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.strong.transformed.expect
@@ -81,14 +81,14 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
    ^^^^^^^^^^^^^^^^" && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:32:7: Error: Can't assign to the final variable '#initializedLateFinal'.
       initializedLateFinal, // Error
       ^^^^^^^^^^^^^^^^^^^^^" && (let final core::Object? #t5 = definitelyUnassignedFinal = #0#0{(core::Object?, core::Object?, core::Object?)}.$3{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t6 = #definitelyUnassignedLateFinal#set(#1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int?){(core::int) → dynamic} in true) && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:35:5: Error: Final variable 'definitelyAssignedFinal' might already be assigned at this point.
     definitelyAssignedFinal] = y; // Error
     ^^^^^^^^^^^^^^^^^^^^^^^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -109,7 +109,7 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#3#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#3#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#9#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:37:17: Error: Final variable 'notDefinitelyAssignedFinal' might already be assigned at this point.
       field2: [[notDefinitelyAssignedFinal, _], // Error
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#16#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#16#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t11 = #notDefinitelyAssignedLateFinal#set((#2#16#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::[]}(1){(core::int) → dynamic} as{ForNonNullableByDefault} core::int?){(core::int) → dynamic} in true)))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.weak.expect b/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.weak.expect
index 797762ca4a7..c44a8fb979e 100644
--- a/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.weak.expect
+++ b/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.weak.expect
@@ -96,14 +96,14 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
    ^^^^^^^^^^^^^^^^" && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:32:7: Error: Can't assign to the final variable '#initializedLateFinal'.
       initializedLateFinal, // Error
       ^^^^^^^^^^^^^^^^^^^^^" && (let final dynamic #t2 = definitelyUnassignedFinal = #0#0{(core::Object?, core::Object?, core::Object?)}.$3{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t3 = #definitelyUnassignedLateFinal#set(#1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int?){(core::int) → dynamic} in true) && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:35:5: Error: Final variable 'definitelyAssignedFinal' might already be assigned at this point.
     definitelyAssignedFinal] = y; // Error
     ^^^^^^^^^^^^^^^^^^^^^^^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -124,7 +124,7 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#3#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#3#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#9#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:37:17: Error: Final variable 'notDefinitelyAssignedFinal' might already be assigned at this point.
       field2: [[notDefinitelyAssignedFinal, _], // Error
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#16#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#16#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t8 = #notDefinitelyAssignedLateFinal#set((#2#16#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::[]}(1){(core::int) → dynamic} as{ForNonNullableByDefault} core::int?){(core::int) → dynamic} in true)))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.weak.modular.expect b/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.weak.modular.expect
index 797762ca4a7..c44a8fb979e 100644
--- a/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.weak.modular.expect
@@ -96,14 +96,14 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
    ^^^^^^^^^^^^^^^^" && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:32:7: Error: Can't assign to the final variable '#initializedLateFinal'.
       initializedLateFinal, // Error
       ^^^^^^^^^^^^^^^^^^^^^" && (let final dynamic #t2 = definitelyUnassignedFinal = #0#0{(core::Object?, core::Object?, core::Object?)}.$3{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t3 = #definitelyUnassignedLateFinal#set(#1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int?){(core::int) → dynamic} in true) && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:35:5: Error: Final variable 'definitelyAssignedFinal' might already be assigned at this point.
     definitelyAssignedFinal] = y; // Error
     ^^^^^^^^^^^^^^^^^^^^^^^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -124,7 +124,7 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#3#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#3#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#9#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:37:17: Error: Final variable 'notDefinitelyAssignedFinal' might already be assigned at this point.
       field2: [[notDefinitelyAssignedFinal, _], // Error
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#16#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#16#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t8 = #notDefinitelyAssignedLateFinal#set((#2#16#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::[]}(1){(core::int) → dynamic} as{ForNonNullableByDefault} core::int?){(core::int) → dynamic} in true)))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.weak.transformed.expect b/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.weak.transformed.expect
index 59795ad5a31..1b2cbd703c0 100644
--- a/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart.weak.transformed.expect
@@ -96,14 +96,14 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
    ^^^^^^^^^^^^^^^^" && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:32:7: Error: Can't assign to the final variable '#initializedLateFinal'.
       initializedLateFinal, // Error
       ^^^^^^^^^^^^^^^^^^^^^" && (let final core::Object? #t2 = definitelyUnassignedFinal = #0#0{(core::Object?, core::Object?, core::Object?)}.$3{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t3 = #definitelyUnassignedLateFinal#set(#1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int?){(core::int) → dynamic} in true) && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:35:5: Error: Final variable 'definitelyAssignedFinal' might already be assigned at this point.
     definitelyAssignedFinal] = y; // Error
     ^^^^^^^^^^^^^^^^^^^^^^^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -124,7 +124,7 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#3#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#3#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#9#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && invalid-expression "pkg/front_end/testcases/late_lowering/pattern_assignment_final.dart:37:17: Error: Final variable 'notDefinitelyAssignedFinal' might already be assigned at this point.
       field2: [[notDefinitelyAssignedFinal, _], // Error
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#16#func(){() → dynamic} is{ForNonNullableByDefault} core::List<dynamic> && (#2#16#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t8 = #notDefinitelyAssignedLateFinal#set((#2#16#func(){() → dynamic} as{ForNonNullableByDefault} core::List<dynamic>).{core::List::[]}(1){(core::int) → dynamic} as{ForNonNullableByDefault} core::int?){(core::int) → dynamic} in true)))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/patterns/dynamic_guard.dart.strong.expect b/pkg/front_end/testcases/patterns/dynamic_guard.dart.strong.expect
index 4753fcf2259..0de2f0638df 100644
--- a/pkg/front_end/testcases/patterns/dynamic_guard.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/dynamic_guard.dart.strong.expect
@@ -1,7 +1,6 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method guard() → dynamic
   return true;
@@ -16,7 +15,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #1#0 = <core::int>[5];
     if(!(#1#0{core::List<core::int>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = b = #1#0{core::List<core::int>}.{core::List::[]}(0){(core::int) → core::int} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   {
     final synthesized core::int #2#0 = a;
diff --git a/pkg/front_end/testcases/patterns/dynamic_guard.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/dynamic_guard.dart.strong.transformed.expect
index c6b42cb04cf..421d9eb87b1 100644
--- a/pkg/front_end/testcases/patterns/dynamic_guard.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/dynamic_guard.dart.strong.transformed.expect
@@ -1,7 +1,6 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method guard() → dynamic
   return true;
@@ -16,7 +15,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #1#0 = core::_GrowableList::_literal1<core::int>(5);
     if(!(#1#0{core::List<core::int>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final core::int #t2 = b = #1#0{core::List<core::int>}.{core::List::[]}(0){(core::int) → core::int} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   {
     final synthesized core::int #2#0 = a;
diff --git a/pkg/front_end/testcases/patterns/dynamic_guard.dart.weak.expect b/pkg/front_end/testcases/patterns/dynamic_guard.dart.weak.expect
index a0a270a0007..9a146cdbc94 100644
--- a/pkg/front_end/testcases/patterns/dynamic_guard.dart.weak.expect
+++ b/pkg/front_end/testcases/patterns/dynamic_guard.dart.weak.expect
@@ -16,7 +16,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #1#0 = <core::int>[5];
     if(!(#1#0{core::List<core::int>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = b = #1#0{core::List<core::int>}.{core::List::[]}(0){(core::int) → core::int} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   {
     final synthesized core::int #2#0 = a;
diff --git a/pkg/front_end/testcases/patterns/dynamic_guard.dart.weak.modular.expect b/pkg/front_end/testcases/patterns/dynamic_guard.dart.weak.modular.expect
index a0a270a0007..9a146cdbc94 100644
--- a/pkg/front_end/testcases/patterns/dynamic_guard.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/patterns/dynamic_guard.dart.weak.modular.expect
@@ -16,7 +16,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #1#0 = <core::int>[5];
     if(!(#1#0{core::List<core::int>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = b = #1#0{core::List<core::int>}.{core::List::[]}(0){(core::int) → core::int} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   {
     final synthesized core::int #2#0 = a;
diff --git a/pkg/front_end/testcases/patterns/dynamic_guard.dart.weak.transformed.expect b/pkg/front_end/testcases/patterns/dynamic_guard.dart.weak.transformed.expect
index 82195a59f02..da657d70dd4 100644
--- a/pkg/front_end/testcases/patterns/dynamic_guard.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/patterns/dynamic_guard.dart.weak.transformed.expect
@@ -16,7 +16,7 @@ static method main() → dynamic {
   block {
     final synthesized dynamic #1#0 = core::_GrowableList::_literal1<core::int>(5);
     if(!(#1#0{core::List<core::int>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final core::int #t2 = b = #1#0{core::List<core::int>}.{core::List::[]}(0){(core::int) → core::int} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   {
     final synthesized core::int #2#0 = a;
diff --git a/pkg/front_end/testcases/patterns/issue51432.dart.strong.expect b/pkg/front_end/testcases/patterns/issue51432.dart.strong.expect
index 86406f424dc..32c97a318b9 100644
--- a/pkg/front_end/testcases/patterns/issue51432.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/issue51432.dart.strong.expect
@@ -1,14 +1,13 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method method() → dynamic {
   core::int v1 = 42;
   block {
     final synthesized dynamic #0#0 = "The string!" as{ForNonNullableByDefault} dynamic;
     if(!(let final dynamic #t1 = v1 = #0#0 as{ForNonNullableByDefault} core::int in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(v1);
   core::print(v1.{core::Object::runtimeType}{core::Type});
diff --git a/pkg/front_end/testcases/patterns/issue51432.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/issue51432.dart.strong.transformed.expect
index 8af5e77fa55..20d9756fdc1 100644
--- a/pkg/front_end/testcases/patterns/issue51432.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/issue51432.dart.strong.transformed.expect
@@ -1,14 +1,13 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method method() → dynamic {
   core::int v1 = 42;
   block {
     final synthesized dynamic #0#0 = "The string!" as{ForNonNullableByDefault} dynamic;
     if(!(let final core::int #t1 = v1 = #0#0 as{ForNonNullableByDefault} core::int in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(v1);
   core::print(v1.{core::Object::runtimeType}{core::Type});
diff --git a/pkg/front_end/testcases/patterns/issue51432.dart.weak.expect b/pkg/front_end/testcases/patterns/issue51432.dart.weak.expect
index 86406f424dc..32c97a318b9 100644
--- a/pkg/front_end/testcases/patterns/issue51432.dart.weak.expect
+++ b/pkg/front_end/testcases/patterns/issue51432.dart.weak.expect
@@ -1,14 +1,13 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method method() → dynamic {
   core::int v1 = 42;
   block {
     final synthesized dynamic #0#0 = "The string!" as{ForNonNullableByDefault} dynamic;
     if(!(let final dynamic #t1 = v1 = #0#0 as{ForNonNullableByDefault} core::int in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(v1);
   core::print(v1.{core::Object::runtimeType}{core::Type});
diff --git a/pkg/front_end/testcases/patterns/issue51432.dart.weak.modular.expect b/pkg/front_end/testcases/patterns/issue51432.dart.weak.modular.expect
index 86406f424dc..32c97a318b9 100644
--- a/pkg/front_end/testcases/patterns/issue51432.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/patterns/issue51432.dart.weak.modular.expect
@@ -1,14 +1,13 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method method() → dynamic {
   core::int v1 = 42;
   block {
     final synthesized dynamic #0#0 = "The string!" as{ForNonNullableByDefault} dynamic;
     if(!(let final dynamic #t1 = v1 = #0#0 as{ForNonNullableByDefault} core::int in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(v1);
   core::print(v1.{core::Object::runtimeType}{core::Type});
diff --git a/pkg/front_end/testcases/patterns/issue51432.dart.weak.transformed.expect b/pkg/front_end/testcases/patterns/issue51432.dart.weak.transformed.expect
index 8af5e77fa55..20d9756fdc1 100644
--- a/pkg/front_end/testcases/patterns/issue51432.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/patterns/issue51432.dart.weak.transformed.expect
@@ -1,14 +1,13 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method method() → dynamic {
   core::int v1 = 42;
   block {
     final synthesized dynamic #0#0 = "The string!" as{ForNonNullableByDefault} dynamic;
     if(!(let final core::int #t1 = v1 = #0#0 as{ForNonNullableByDefault} core::int in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   core::print(v1);
   core::print(v1.{core::Object::runtimeType}{core::Type});
diff --git a/pkg/front_end/testcases/patterns/issue51587.dart.strong.expect b/pkg/front_end/testcases/patterns/issue51587.dart.strong.expect
index 66f1d9ccdf4..391592daa89 100644
--- a/pkg/front_end/testcases/patterns/issue51587.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/issue51587.dart.strong.expect
@@ -1,7 +1,6 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method test1() → dynamic {
   core::num b = 0;
@@ -10,7 +9,7 @@ static method test1() → dynamic {
   block {
     final synthesized dynamic #0#0 = (3.14);
     if(!(let final dynamic #t1 = b = #0#0{(core::double)}.$1{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
@@ -21,7 +20,7 @@ static method test2() → dynamic {
   block {
     final synthesized dynamic #0#0 = (3.14, {foo: "foo"});
     if(!(let final dynamic #t2 = b = #0#0{(core::double, {foo: core::String})}.$1{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
@@ -32,7 +31,7 @@ static method test3() → dynamic {
   block {
     final synthesized dynamic #0#0 = ({foo: 3.14});
     if(!(let final dynamic #t3 = b = #0#0{({foo: core::double})}.foo{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
diff --git a/pkg/front_end/testcases/patterns/issue51587.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/issue51587.dart.strong.transformed.expect
index 6384861e38c..7208602ae0d 100644
--- a/pkg/front_end/testcases/patterns/issue51587.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/issue51587.dart.strong.transformed.expect
@@ -1,7 +1,6 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method test1() → dynamic {
   core::num b = 0;
@@ -10,7 +9,7 @@ static method test1() → dynamic {
   block {
     final synthesized dynamic #0#0 = (3.14);
     if(!(let final core::double #t1 = b = #0#0{(core::double)}.$1{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
@@ -21,7 +20,7 @@ static method test2() → dynamic {
   block {
     final synthesized dynamic #0#0 = (3.14, {foo: "foo"});
     if(!(let final core::double #t2 = b = #0#0{(core::double, {foo: core::String})}.$1{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
@@ -32,7 +31,7 @@ static method test3() → dynamic {
   block {
     final synthesized dynamic #0#0 = ({foo: 3.14});
     if(!(let final core::double #t3 = b = #0#0{({foo: core::double})}.foo{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
diff --git a/pkg/front_end/testcases/patterns/issue51587.dart.weak.expect b/pkg/front_end/testcases/patterns/issue51587.dart.weak.expect
index 66f1d9ccdf4..391592daa89 100644
--- a/pkg/front_end/testcases/patterns/issue51587.dart.weak.expect
+++ b/pkg/front_end/testcases/patterns/issue51587.dart.weak.expect
@@ -1,7 +1,6 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method test1() → dynamic {
   core::num b = 0;
@@ -10,7 +9,7 @@ static method test1() → dynamic {
   block {
     final synthesized dynamic #0#0 = (3.14);
     if(!(let final dynamic #t1 = b = #0#0{(core::double)}.$1{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
@@ -21,7 +20,7 @@ static method test2() → dynamic {
   block {
     final synthesized dynamic #0#0 = (3.14, {foo: "foo"});
     if(!(let final dynamic #t2 = b = #0#0{(core::double, {foo: core::String})}.$1{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
@@ -32,7 +31,7 @@ static method test3() → dynamic {
   block {
     final synthesized dynamic #0#0 = ({foo: 3.14});
     if(!(let final dynamic #t3 = b = #0#0{({foo: core::double})}.foo{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
diff --git a/pkg/front_end/testcases/patterns/issue51587.dart.weak.modular.expect b/pkg/front_end/testcases/patterns/issue51587.dart.weak.modular.expect
index 66f1d9ccdf4..391592daa89 100644
--- a/pkg/front_end/testcases/patterns/issue51587.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/patterns/issue51587.dart.weak.modular.expect
@@ -1,7 +1,6 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method test1() → dynamic {
   core::num b = 0;
@@ -10,7 +9,7 @@ static method test1() → dynamic {
   block {
     final synthesized dynamic #0#0 = (3.14);
     if(!(let final dynamic #t1 = b = #0#0{(core::double)}.$1{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
@@ -21,7 +20,7 @@ static method test2() → dynamic {
   block {
     final synthesized dynamic #0#0 = (3.14, {foo: "foo"});
     if(!(let final dynamic #t2 = b = #0#0{(core::double, {foo: core::String})}.$1{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
@@ -32,7 +31,7 @@ static method test3() → dynamic {
   block {
     final synthesized dynamic #0#0 = ({foo: 3.14});
     if(!(let final dynamic #t3 = b = #0#0{({foo: core::double})}.foo{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
diff --git a/pkg/front_end/testcases/patterns/issue51587.dart.weak.transformed.expect b/pkg/front_end/testcases/patterns/issue51587.dart.weak.transformed.expect
index 6384861e38c..7208602ae0d 100644
--- a/pkg/front_end/testcases/patterns/issue51587.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/patterns/issue51587.dart.weak.transformed.expect
@@ -1,7 +1,6 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method test1() → dynamic {
   core::num b = 0;
@@ -10,7 +9,7 @@ static method test1() → dynamic {
   block {
     final synthesized dynamic #0#0 = (3.14);
     if(!(let final core::double #t1 = b = #0#0{(core::double)}.$1{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
@@ -21,7 +20,7 @@ static method test2() → dynamic {
   block {
     final synthesized dynamic #0#0 = (3.14, {foo: "foo"});
     if(!(let final core::double #t2 = b = #0#0{(core::double, {foo: core::String})}.$1{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
@@ -32,7 +31,7 @@ static method test3() → dynamic {
   block {
     final synthesized dynamic #0#0 = ({foo: 3.14});
     if(!(let final core::double #t3 = b = #0#0{({foo: core::double})}.foo{core::double} in true))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return b;
 }
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.expect b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.expect
index ed6ad28f049..582ce513b84 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.expect
@@ -34,7 +34,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   field dynamic field1 = null;
@@ -54,7 +53,7 @@ Try using a preexisting variable or changing the assignment to a pattern variabl
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   (a, int b, final int c) = x;
                        ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
@@ -62,7 +61,7 @@ Try using a preexisting variable or changing the assignment to a pattern variabl
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   [a, final d] = y;
             ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -79,7 +78,7 @@ Try using an if-case, a 'switch' statement, or a 'switch' expression instead.
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
                                                 ^"))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.transformed.expect
index 2b460b366eb..c60b34a54e6 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.transformed.expect
@@ -34,7 +34,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   field dynamic field1 = null;
@@ -54,7 +53,7 @@ Try using a preexisting variable or changing the assignment to a pattern variabl
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   (a, int b, final int c) = x;
                        ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
@@ -62,7 +61,7 @@ Try using a preexisting variable or changing the assignment to a pattern variabl
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   [a, final d] = y;
             ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -85,7 +84,7 @@ Try using an if-case, a 'switch' statement, or a 'switch' expression instead.
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
                                                 ^"))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.expect b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.expect
index ed6ad28f049..582ce513b84 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.expect
@@ -34,7 +34,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   field dynamic field1 = null;
@@ -54,7 +53,7 @@ Try using a preexisting variable or changing the assignment to a pattern variabl
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   (a, int b, final int c) = x;
                        ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
@@ -62,7 +61,7 @@ Try using a preexisting variable or changing the assignment to a pattern variabl
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   [a, final d] = y;
             ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -79,7 +78,7 @@ Try using an if-case, a 'switch' statement, or a 'switch' expression instead.
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
                                                 ^"))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.modular.expect b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.modular.expect
index ed6ad28f049..582ce513b84 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.modular.expect
@@ -34,7 +34,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   field dynamic field1 = null;
@@ -54,7 +53,7 @@ Try using a preexisting variable or changing the assignment to a pattern variabl
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   (a, int b, final int c) = x;
                        ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
@@ -62,7 +61,7 @@ Try using a preexisting variable or changing the assignment to a pattern variabl
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   [a, final d] = y;
             ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -79,7 +78,7 @@ Try using an if-case, a 'switch' statement, or a 'switch' expression instead.
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
                                                 ^"))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.transformed.expect b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.transformed.expect
index 2b460b366eb..c60b34a54e6 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.transformed.expect
@@ -34,7 +34,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   field dynamic field1 = null;
@@ -54,7 +53,7 @@ Try using a preexisting variable or changing the assignment to a pattern variabl
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   (a, int b, final int c) = x;
                        ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
@@ -62,7 +61,7 @@ Try using a preexisting variable or changing the assignment to a pattern variabl
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   [a, final d] = y;
             ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -85,7 +84,7 @@ Try using an if-case, a 'switch' statement, or a 'switch' expression instead.
 Try using a preexisting variable or changing the assignment to a pattern variable declaration.
   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
                                                 ^"))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.strong.expect b/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.strong.expect
index 052964bd2fe..8414c189f45 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.strong.expect
@@ -24,7 +24,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   field dynamic field1 = null;
@@ -55,14 +54,14 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
    ^^^^^^^^^^^^^^^^" && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:32:7: Error: Late final variable 'initializedLateFinal' definitely assigned.
       initializedLateFinal, // Error
       ^^^^^^^^^^^^^^^^^^^^" && (let final dynamic #t1 = definitelyUnassignedFinal = #0#0{(core::Object?, core::Object?, core::Object?)}.$3{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = definitelyUnassignedLateFinal = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true) && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:35:5: Error: Final variable 'definitelyAssignedFinal' might already be assigned at this point.
     definitelyAssignedFinal] = y; // Error
     ^^^^^^^^^^^^^^^^^^^^^^^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -74,7 +73,7 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#3 is{ForNonNullableByDefault} core::List<dynamic> && #2#3{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9 is{ForNonNullableByDefault} core::List<dynamic> && #2#9{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:37:17: Error: Final variable 'notDefinitelyAssignedFinal' might already be assigned at this point.
       field2: [[notDefinitelyAssignedFinal, _], // Error
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#16 is{ForNonNullableByDefault} core::List<dynamic> && #2#16{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t4 = notDefinitelyAssignedLateFinal = #2#16{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.strong.transformed.expect
index 51361a908c0..9109509ae8b 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.strong.transformed.expect
@@ -24,7 +24,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   field dynamic field1 = null;
@@ -55,14 +54,14 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
    ^^^^^^^^^^^^^^^^" && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:32:7: Error: Late final variable 'initializedLateFinal' definitely assigned.
       initializedLateFinal, // Error
       ^^^^^^^^^^^^^^^^^^^^" && (let final core::Object? #t1 = definitelyUnassignedFinal = #0#0{(core::Object?, core::Object?, core::Object?)}.$3{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final core::int #t2 = definitelyUnassignedLateFinal = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true) && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:35:5: Error: Final variable 'definitelyAssignedFinal' might already be assigned at this point.
     definitelyAssignedFinal] = y; // Error
     ^^^^^^^^^^^^^^^^^^^^^^^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -80,7 +79,7 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#3 is{ForNonNullableByDefault} core::List<dynamic> && #2#3{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9 is{ForNonNullableByDefault} core::List<dynamic> && #2#9{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:37:17: Error: Final variable 'notDefinitelyAssignedFinal' might already be assigned at this point.
       field2: [[notDefinitelyAssignedFinal, _], // Error
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#16 is{ForNonNullableByDefault} core::List<dynamic> && #2#16{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final core::int #t4 = notDefinitelyAssignedLateFinal = #2#16{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.weak.expect b/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.weak.expect
index 052964bd2fe..8414c189f45 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.weak.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.weak.expect
@@ -24,7 +24,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   field dynamic field1 = null;
@@ -55,14 +54,14 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
    ^^^^^^^^^^^^^^^^" && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:32:7: Error: Late final variable 'initializedLateFinal' definitely assigned.
       initializedLateFinal, // Error
       ^^^^^^^^^^^^^^^^^^^^" && (let final dynamic #t1 = definitelyUnassignedFinal = #0#0{(core::Object?, core::Object?, core::Object?)}.$3{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = definitelyUnassignedLateFinal = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true) && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:35:5: Error: Final variable 'definitelyAssignedFinal' might already be assigned at this point.
     definitelyAssignedFinal] = y; // Error
     ^^^^^^^^^^^^^^^^^^^^^^^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -74,7 +73,7 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#3 is{ForNonNullableByDefault} core::List<dynamic> && #2#3{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9 is{ForNonNullableByDefault} core::List<dynamic> && #2#9{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:37:17: Error: Final variable 'notDefinitelyAssignedFinal' might already be assigned at this point.
       field2: [[notDefinitelyAssignedFinal, _], // Error
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#16 is{ForNonNullableByDefault} core::List<dynamic> && #2#16{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t4 = notDefinitelyAssignedLateFinal = #2#16{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.weak.modular.expect b/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.weak.modular.expect
index 052964bd2fe..8414c189f45 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.weak.modular.expect
@@ -24,7 +24,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   field dynamic field1 = null;
@@ -55,14 +54,14 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
    ^^^^^^^^^^^^^^^^" && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:32:7: Error: Late final variable 'initializedLateFinal' definitely assigned.
       initializedLateFinal, // Error
       ^^^^^^^^^^^^^^^^^^^^" && (let final dynamic #t1 = definitelyUnassignedFinal = #0#0{(core::Object?, core::Object?, core::Object?)}.$3{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = definitelyUnassignedLateFinal = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true) && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:35:5: Error: Final variable 'definitelyAssignedFinal' might already be assigned at this point.
     definitelyAssignedFinal] = y; // Error
     ^^^^^^^^^^^^^^^^^^^^^^^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -74,7 +73,7 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#3 is{ForNonNullableByDefault} core::List<dynamic> && #2#3{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9 is{ForNonNullableByDefault} core::List<dynamic> && #2#9{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:37:17: Error: Final variable 'notDefinitelyAssignedFinal' might already be assigned at this point.
       field2: [[notDefinitelyAssignedFinal, _], // Error
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#16 is{ForNonNullableByDefault} core::List<dynamic> && #2#16{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t4 = notDefinitelyAssignedLateFinal = #2#16{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.weak.transformed.expect b/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.weak.transformed.expect
index 51361a908c0..9109509ae8b 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_final.dart.weak.transformed.expect
@@ -24,7 +24,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   field dynamic field1 = null;
@@ -55,14 +54,14 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
    ^^^^^^^^^^^^^^^^" && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:32:7: Error: Late final variable 'initializedLateFinal' definitely assigned.
       initializedLateFinal, // Error
       ^^^^^^^^^^^^^^^^^^^^" && (let final core::Object? #t1 = definitelyUnassignedFinal = #0#0{(core::Object?, core::Object?, core::Object?)}.$3{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   block {
     final synthesized dynamic #1#0 = y;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final core::int #t2 = definitelyUnassignedLateFinal = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true) && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:35:5: Error: Final variable 'definitelyAssignedFinal' might already be assigned at this point.
     definitelyAssignedFinal] = y; // Error
     ^^^^^^^^^^^^^^^^^^^^^^^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   block {
     final synthesized dynamic #2#0 = z;
@@ -80,7 +79,7 @@ static method method(dynamic x, dynamic y, dynamic z) → dynamic {
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#3 is{ForNonNullableByDefault} core::List<dynamic> && #2#3{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9 is{ForNonNullableByDefault} core::List<dynamic> && #2#9{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_final.dart:37:17: Error: Final variable 'notDefinitelyAssignedFinal' might already be assigned at this point.
       field2: [[notDefinitelyAssignedFinal, _], // Error
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^") && (#2#16 is{ForNonNullableByDefault} core::List<dynamic> && #2#16{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final core::int #t4 = notDefinitelyAssignedLateFinal = #2#16{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#2#0;
 }
 
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.strong.expect b/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.strong.expect
index ef425cc7791..10849b1ac82 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.strong.expect
@@ -24,7 +24,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Super extends core::Object {
   field dynamic superField = null;
@@ -43,7 +42,7 @@ class Class extends self::Super {
     block {
       final synthesized dynamic #0#0 = o;
       if(!(#0#0 is{ForNonNullableByDefault} (core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?) && (let final dynamic #t1 = local = #0#0{(core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?)}.$1{core::Object?} in true) && (let final dynamic #t2 = parameter = #0#0{(core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?)}.$2{core::Object?} in true)))
-        throw new _in::ReachabilityError::•();
+        throw new core::StateError::•("Pattern matching error");
     } =>#0#0;
   }
 }
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.strong.transformed.expect
index 27a907bb3a4..e23645db45c 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.strong.transformed.expect
@@ -24,7 +24,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Super extends core::Object {
   field dynamic superField = null;
@@ -43,7 +42,7 @@ class Class extends self::Super {
     block {
       final synthesized dynamic #0#0 = o;
       if(!(#0#0 is{ForNonNullableByDefault} (core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?) && (let final core::Object? #t1 = local = #0#0{(core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?)}.$1{core::Object?} in true) && (let final core::Object? #t2 = parameter = #0#0{(core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?)}.$2{core::Object?} in true)))
-        throw new _in::ReachabilityError::•();
+        throw new core::StateError::•("Pattern matching error");
     } =>#0#0;
   }
 }
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.weak.expect b/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.weak.expect
index ef425cc7791..10849b1ac82 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.weak.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.weak.expect
@@ -24,7 +24,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Super extends core::Object {
   field dynamic superField = null;
@@ -43,7 +42,7 @@ class Class extends self::Super {
     block {
       final synthesized dynamic #0#0 = o;
       if(!(#0#0 is{ForNonNullableByDefault} (core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?) && (let final dynamic #t1 = local = #0#0{(core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?)}.$1{core::Object?} in true) && (let final dynamic #t2 = parameter = #0#0{(core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?)}.$2{core::Object?} in true)))
-        throw new _in::ReachabilityError::•();
+        throw new core::StateError::•("Pattern matching error");
     } =>#0#0;
   }
 }
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.weak.modular.expect b/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.weak.modular.expect
index ef425cc7791..10849b1ac82 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.weak.modular.expect
@@ -24,7 +24,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Super extends core::Object {
   field dynamic superField = null;
@@ -43,7 +42,7 @@ class Class extends self::Super {
     block {
       final synthesized dynamic #0#0 = o;
       if(!(#0#0 is{ForNonNullableByDefault} (core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?) && (let final dynamic #t1 = local = #0#0{(core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?)}.$1{core::Object?} in true) && (let final dynamic #t2 = parameter = #0#0{(core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?)}.$2{core::Object?} in true)))
-        throw new _in::ReachabilityError::•();
+        throw new core::StateError::•("Pattern matching error");
     } =>#0#0;
   }
 }
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.weak.transformed.expect b/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.weak.transformed.expect
index 27a907bb3a4..e23645db45c 100644
--- a/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_non_variable.dart.weak.transformed.expect
@@ -24,7 +24,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Super extends core::Object {
   field dynamic superField = null;
@@ -43,7 +42,7 @@ class Class extends self::Super {
     block {
       final synthesized dynamic #0#0 = o;
       if(!(#0#0 is{ForNonNullableByDefault} (core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?) && (let final core::Object? #t1 = local = #0#0{(core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?)}.$1{core::Object?} in true) && (let final core::Object? #t2 = parameter = #0#0{(core::Object?, core::Object?, core::Object?, core::Object?, core::Object?, core::Object?)}.$2{core::Object?} in true)))
-        throw new _in::ReachabilityError::•();
+        throw new core::StateError::•("Pattern matching error");
     } =>#0#0;
   }
 }
diff --git a/pkg/front_end/testcases/patterns/shared_errors.dart.strong.expect b/pkg/front_end/testcases/patterns/shared_errors.dart.strong.expect
index 13fcb18446b..23c20cd0208 100644
--- a/pkg/front_end/testcases/patterns/shared_errors.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/shared_errors.dart.strong.expect
@@ -89,7 +89,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   synthetic constructor •() → self::Class
@@ -154,7 +153,7 @@ static method duplicateAssignmentPatternVariable(core::List<core::String> list)
 Try renaming the variable.
   [a, a] = list; // Error
       ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
 }
 static method duplicateRecordPatternField(dynamic o) → dynamic {
diff --git a/pkg/front_end/testcases/patterns/shared_errors.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/shared_errors.dart.strong.transformed.expect
index adbad0368bd..1a0677eb6b4 100644
--- a/pkg/front_end/testcases/patterns/shared_errors.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/shared_errors.dart.strong.transformed.expect
@@ -89,7 +89,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   synthetic constructor •() → self::Class
@@ -154,7 +153,7 @@ static method duplicateAssignmentPatternVariable(core::List<core::String> list)
 Try renaming the variable.
   [a, a] = list; // Error
       ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
 }
 static method duplicateRecordPatternField(dynamic o) → dynamic {
diff --git a/pkg/front_end/testcases/patterns/shared_errors.dart.weak.expect b/pkg/front_end/testcases/patterns/shared_errors.dart.weak.expect
index 13fcb18446b..23c20cd0208 100644
--- a/pkg/front_end/testcases/patterns/shared_errors.dart.weak.expect
+++ b/pkg/front_end/testcases/patterns/shared_errors.dart.weak.expect
@@ -89,7 +89,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   synthetic constructor •() → self::Class
@@ -154,7 +153,7 @@ static method duplicateAssignmentPatternVariable(core::List<core::String> list)
 Try renaming the variable.
   [a, a] = list; // Error
       ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
 }
 static method duplicateRecordPatternField(dynamic o) → dynamic {
diff --git a/pkg/front_end/testcases/patterns/shared_errors.dart.weak.modular.expect b/pkg/front_end/testcases/patterns/shared_errors.dart.weak.modular.expect
index 13fcb18446b..23c20cd0208 100644
--- a/pkg/front_end/testcases/patterns/shared_errors.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/patterns/shared_errors.dart.weak.modular.expect
@@ -89,7 +89,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   synthetic constructor •() → self::Class
@@ -154,7 +153,7 @@ static method duplicateAssignmentPatternVariable(core::List<core::String> list)
 Try renaming the variable.
   [a, a] = list; // Error
       ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
 }
 static method duplicateRecordPatternField(dynamic o) → dynamic {
diff --git a/pkg/front_end/testcases/patterns/shared_errors.dart.weak.transformed.expect b/pkg/front_end/testcases/patterns/shared_errors.dart.weak.transformed.expect
index adbad0368bd..1a0677eb6b4 100644
--- a/pkg/front_end/testcases/patterns/shared_errors.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/patterns/shared_errors.dart.weak.transformed.expect
@@ -89,7 +89,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 class Class extends core::Object {
   synthetic constructor •() → self::Class
@@ -154,7 +153,7 @@ static method duplicateAssignmentPatternVariable(core::List<core::String> list)
 Try renaming the variable.
   [a, a] = list; // Error
       ^"))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
 }
 static method duplicateRecordPatternField(dynamic o) → dynamic {
diff --git a/pkg/front_end/testcases/patterns/side_effect.dart.strong.expect b/pkg/front_end/testcases/patterns/side_effect.dart.strong.expect
index 002d7b2dff3..bfbd1adcc42 100644
--- a/pkg/front_end/testcases/patterns/side_effect.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/side_effect.dart.strong.expect
@@ -1,7 +1,6 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method sideEffect<T extends core::Object? = dynamic>(core::List<self::sideEffect::T%> list) → core::List<self::sideEffect::T%> {
   core::print(self::sideEffect::T%);
@@ -16,7 +15,7 @@ static method method() → dynamic {
   block {
     final synthesized dynamic #1#0 = self::sideEffect<dynamic>(<dynamic>[]);
     if(!true)
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   {
     final synthesized core::List<dynamic> #2#0 = self::sideEffect<dynamic>(<dynamic>[]);
diff --git a/pkg/front_end/testcases/patterns/side_effect.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/side_effect.dart.strong.transformed.expect
index bb093a4c80b..138e21ab4c7 100644
--- a/pkg/front_end/testcases/patterns/side_effect.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/side_effect.dart.strong.transformed.expect
@@ -1,7 +1,6 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method sideEffect<T extends core::Object? = dynamic>(core::List<self::sideEffect::T%> list) → core::List<self::sideEffect::T%> {
   core::print(self::sideEffect::T%);
@@ -16,7 +15,7 @@ static method method() → dynamic {
   block {
     final synthesized dynamic #1#0 = self::sideEffect<dynamic>(core::_GrowableList::•<dynamic>(0));
     if(!true)
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   {
     final synthesized core::List<dynamic> #2#0 = self::sideEffect<dynamic>(core::_GrowableList::•<dynamic>(0));
diff --git a/pkg/front_end/testcases/patterns/side_effect.dart.weak.expect b/pkg/front_end/testcases/patterns/side_effect.dart.weak.expect
index f2c879ecc88..bafc2224ed2 100644
--- a/pkg/front_end/testcases/patterns/side_effect.dart.weak.expect
+++ b/pkg/front_end/testcases/patterns/side_effect.dart.weak.expect
@@ -16,7 +16,7 @@ static method method() → dynamic {
   block {
     final synthesized dynamic #1#0 = self::sideEffect<dynamic>(<dynamic>[]);
     if(!true)
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   {
     final synthesized core::List<dynamic> #2#0 = self::sideEffect<dynamic>(<dynamic>[]);
diff --git a/pkg/front_end/testcases/patterns/side_effect.dart.weak.modular.expect b/pkg/front_end/testcases/patterns/side_effect.dart.weak.modular.expect
index f2c879ecc88..bafc2224ed2 100644
--- a/pkg/front_end/testcases/patterns/side_effect.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/patterns/side_effect.dart.weak.modular.expect
@@ -16,7 +16,7 @@ static method method() → dynamic {
   block {
     final synthesized dynamic #1#0 = self::sideEffect<dynamic>(<dynamic>[]);
     if(!true)
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   {
     final synthesized core::List<dynamic> #2#0 = self::sideEffect<dynamic>(<dynamic>[]);
diff --git a/pkg/front_end/testcases/patterns/side_effect.dart.weak.transformed.expect b/pkg/front_end/testcases/patterns/side_effect.dart.weak.transformed.expect
index 91b47630441..b680c2fb0db 100644
--- a/pkg/front_end/testcases/patterns/side_effect.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/patterns/side_effect.dart.weak.transformed.expect
@@ -16,7 +16,7 @@ static method method() → dynamic {
   block {
     final synthesized dynamic #1#0 = self::sideEffect<dynamic>(core::_GrowableList::•<dynamic>(0));
     if(!true)
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   {
     final synthesized core::List<dynamic> #2#0 = self::sideEffect<dynamic>(core::_GrowableList::•<dynamic>(0));
diff --git a/pkg/front_end/testcases/patterns/simple_assignments.dart.strong.expect b/pkg/front_end/testcases/patterns/simple_assignments.dart.strong.expect
index 5a01d51229b..979f972245b 100644
--- a/pkg/front_end/testcases/patterns/simple_assignments.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/simple_assignments.dart.strong.expect
@@ -1,14 +1,13 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method test1(dynamic x) → dynamic {
   core::int a;
   block {
     final synthesized dynamic #0#0 = x;
     if(!(#0#0 is{ForNonNullableByDefault} core::List<dynamic> && #0#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t1 = a = #0#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return a;
 }
@@ -21,10 +20,10 @@ static method test2(dynamic x) → dynamic {
       final synthesized dynamic #0#0 = x;
       late final synthesized dynamic #0#6 = #0#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic};
       if(!(#0#0 is{ForNonNullableByDefault} core::List<dynamic> && #0#0{core::List<dynamic>}.{core::List::length}{core::int}.{core::num::>=}(#C1){(core::num) → core::bool} && ((let final dynamic #t2 = a = #0#6 as{ForNonNullableByDefault} core::int in true) && (let final dynamic #t3 = b = #0#6{core::int} in true))))
-        throw new _in::ReachabilityError::•();
+        throw new core::StateError::•("Pattern matching error");
     } =>#0#0;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int}.{core::num::>=}(#C1){(core::num) → core::bool} && (let final dynamic #t4 = c = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   return a.{core::num::+}(b){(core::num) → core::int}.{core::num::+}(c){(core::num) → core::int};
 }
diff --git a/pkg/front_end/testcases/patterns/simple_assignments.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/simple_assignments.dart.strong.transformed.expect
index c67e912c62b..cc4e464f5d5 100644
--- a/pkg/front_end/testcases/patterns/simple_assignments.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/simple_assignments.dart.strong.transformed.expect
@@ -1,14 +1,13 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method test1(dynamic x) → dynamic {
   core::int a;
   block {
     final synthesized dynamic #0#0 = x;
     if(!(#0#0 is{ForNonNullableByDefault} core::List<dynamic> && #0#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final core::int #t1 = a = #0#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return a;
 }
@@ -23,10 +22,10 @@ static method test2(dynamic x) → dynamic {
         return #0#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic};
       late final synthesized dynamic #0#6 = ##0#6#initializer(){() → dynamic};
       if(!(#0#0 is{ForNonNullableByDefault} core::List<dynamic> && #0#0{core::List<dynamic>}.{core::List::length}{core::int}.{core::num::>=}(#C1){(core::num) → core::bool} && ((let final core::int #t2 = a = #0#6 as{ForNonNullableByDefault} core::int in true) && (let final core::int #t3 = b = #0#6{core::int} in true))))
-        throw new _in::ReachabilityError::•();
+        throw new core::StateError::•("Pattern matching error");
     } =>#0#0;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int}.{core::num::>=}(#C1){(core::num) → core::bool} && (let final core::int #t4 = c = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   return a.{core::num::+}(b){(core::num) → core::int}.{core::num::+}(c){(core::num) → core::int};
 }
diff --git a/pkg/front_end/testcases/patterns/simple_assignments.dart.weak.expect b/pkg/front_end/testcases/patterns/simple_assignments.dart.weak.expect
index 5a01d51229b..979f972245b 100644
--- a/pkg/front_end/testcases/patterns/simple_assignments.dart.weak.expect
+++ b/pkg/front_end/testcases/patterns/simple_assignments.dart.weak.expect
@@ -1,14 +1,13 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method test1(dynamic x) → dynamic {
   core::int a;
   block {
     final synthesized dynamic #0#0 = x;
     if(!(#0#0 is{ForNonNullableByDefault} core::List<dynamic> && #0#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t1 = a = #0#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return a;
 }
@@ -21,10 +20,10 @@ static method test2(dynamic x) → dynamic {
       final synthesized dynamic #0#0 = x;
       late final synthesized dynamic #0#6 = #0#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic};
       if(!(#0#0 is{ForNonNullableByDefault} core::List<dynamic> && #0#0{core::List<dynamic>}.{core::List::length}{core::int}.{core::num::>=}(#C1){(core::num) → core::bool} && ((let final dynamic #t2 = a = #0#6 as{ForNonNullableByDefault} core::int in true) && (let final dynamic #t3 = b = #0#6{core::int} in true))))
-        throw new _in::ReachabilityError::•();
+        throw new core::StateError::•("Pattern matching error");
     } =>#0#0;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int}.{core::num::>=}(#C1){(core::num) → core::bool} && (let final dynamic #t4 = c = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   return a.{core::num::+}(b){(core::num) → core::int}.{core::num::+}(c){(core::num) → core::int};
 }
diff --git a/pkg/front_end/testcases/patterns/simple_assignments.dart.weak.modular.expect b/pkg/front_end/testcases/patterns/simple_assignments.dart.weak.modular.expect
index 5a01d51229b..979f972245b 100644
--- a/pkg/front_end/testcases/patterns/simple_assignments.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/patterns/simple_assignments.dart.weak.modular.expect
@@ -1,14 +1,13 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method test1(dynamic x) → dynamic {
   core::int a;
   block {
     final synthesized dynamic #0#0 = x;
     if(!(#0#0 is{ForNonNullableByDefault} core::List<dynamic> && #0#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t1 = a = #0#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return a;
 }
@@ -21,10 +20,10 @@ static method test2(dynamic x) → dynamic {
       final synthesized dynamic #0#0 = x;
       late final synthesized dynamic #0#6 = #0#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic};
       if(!(#0#0 is{ForNonNullableByDefault} core::List<dynamic> && #0#0{core::List<dynamic>}.{core::List::length}{core::int}.{core::num::>=}(#C1){(core::num) → core::bool} && ((let final dynamic #t2 = a = #0#6 as{ForNonNullableByDefault} core::int in true) && (let final dynamic #t3 = b = #0#6{core::int} in true))))
-        throw new _in::ReachabilityError::•();
+        throw new core::StateError::•("Pattern matching error");
     } =>#0#0;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int}.{core::num::>=}(#C1){(core::num) → core::bool} && (let final dynamic #t4 = c = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   return a.{core::num::+}(b){(core::num) → core::int}.{core::num::+}(c){(core::num) → core::int};
 }
diff --git a/pkg/front_end/testcases/patterns/simple_assignments.dart.weak.transformed.expect b/pkg/front_end/testcases/patterns/simple_assignments.dart.weak.transformed.expect
index c67e912c62b..cc4e464f5d5 100644
--- a/pkg/front_end/testcases/patterns/simple_assignments.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/patterns/simple_assignments.dart.weak.transformed.expect
@@ -1,14 +1,13 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 static method test1(dynamic x) → dynamic {
   core::int a;
   block {
     final synthesized dynamic #0#0 = x;
     if(!(#0#0 is{ForNonNullableByDefault} core::List<dynamic> && #0#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final core::int #t1 = a = #0#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
   return a;
 }
@@ -23,10 +22,10 @@ static method test2(dynamic x) → dynamic {
         return #0#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic};
       late final synthesized dynamic #0#6 = ##0#6#initializer(){() → dynamic};
       if(!(#0#0 is{ForNonNullableByDefault} core::List<dynamic> && #0#0{core::List<dynamic>}.{core::List::length}{core::int}.{core::num::>=}(#C1){(core::num) → core::bool} && ((let final core::int #t2 = a = #0#6 as{ForNonNullableByDefault} core::int in true) && (let final core::int #t3 = b = #0#6{core::int} in true))))
-        throw new _in::ReachabilityError::•();
+        throw new core::StateError::•("Pattern matching error");
     } =>#0#0;
     if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int}.{core::num::>=}(#C1){(core::num) → core::bool} && (let final core::int #t4 = c = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} as{ForNonNullableByDefault} core::int in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#1#0;
   return a.{core::num::+}(b){(core::num) → core::int}.{core::num::+}(c){(core::num) → core::int};
 }
diff --git a/pkg/front_end/testcases/patterns/variable_names.dart.strong.expect b/pkg/front_end/testcases/patterns/variable_names.dart.strong.expect
index 5ee5b071bd5..8d214604f3a 100644
--- a/pkg/front_end/testcases/patterns/variable_names.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/variable_names.dart.strong.expect
@@ -12,7 +12,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 abstract class A extends core::Object {
   synthetic constructor •() → self::A
@@ -74,11 +73,11 @@ static method test(dynamic x, dynamic y, dynamic z) → dynamic {
   block {
     final synthesized dynamic #4#0 = y;
     if(!(#4#0 is{ForNonNullableByDefault} self::A && (let final dynamic #t15 = qux = #4#0{self::A}.{self::A::qux}{core::bool} in true) && (let final dynamic #t16 = quux = #4#0{self::A}.{self::A::quux}{core::String} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#4#0;
   block {
     final synthesized dynamic #5#0 = z;
     if(!(#5#0 is{ForNonNullableByDefault} (core::Object?, {quux: core::Object?}) && (let final dynamic #t17 = qux = #5#0{(core::Object?, {quux: core::Object?})}.$1{core::Object?} in true) && (let final dynamic #t18 = quux = #5#0{(core::Object?, {quux: core::Object?})}.quux{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#5#0;
 }
diff --git a/pkg/front_end/testcases/patterns/variable_names.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/variable_names.dart.strong.transformed.expect
index 7c31ba40fda..9f94ace94c8 100644
--- a/pkg/front_end/testcases/patterns/variable_names.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/variable_names.dart.strong.transformed.expect
@@ -12,7 +12,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 abstract class A extends core::Object {
   synthetic constructor •() → self::A
@@ -86,11 +85,11 @@ static method test(dynamic x, dynamic y, dynamic z) → dynamic {
   block {
     final synthesized dynamic #4#0 = y;
     if(!(#4#0 is{ForNonNullableByDefault} self::A && (let final core::bool #t15 = qux = #4#0{self::A}.{self::A::qux}{core::bool} in true) && (let final core::String #t16 = quux = #4#0{self::A}.{self::A::quux}{core::String} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#4#0;
   block {
     final synthesized dynamic #5#0 = z;
     if(!(#5#0 is{ForNonNullableByDefault} (core::Object?, {quux: core::Object?}) && (let final core::Object? #t17 = qux = #5#0{(core::Object?, {quux: core::Object?})}.$1{core::Object?} in true) && (let final core::Object? #t18 = quux = #5#0{(core::Object?, {quux: core::Object?})}.quux{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#5#0;
 }
diff --git a/pkg/front_end/testcases/patterns/variable_names.dart.weak.expect b/pkg/front_end/testcases/patterns/variable_names.dart.weak.expect
index 5ee5b071bd5..8d214604f3a 100644
--- a/pkg/front_end/testcases/patterns/variable_names.dart.weak.expect
+++ b/pkg/front_end/testcases/patterns/variable_names.dart.weak.expect
@@ -12,7 +12,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 abstract class A extends core::Object {
   synthetic constructor •() → self::A
@@ -74,11 +73,11 @@ static method test(dynamic x, dynamic y, dynamic z) → dynamic {
   block {
     final synthesized dynamic #4#0 = y;
     if(!(#4#0 is{ForNonNullableByDefault} self::A && (let final dynamic #t15 = qux = #4#0{self::A}.{self::A::qux}{core::bool} in true) && (let final dynamic #t16 = quux = #4#0{self::A}.{self::A::quux}{core::String} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#4#0;
   block {
     final synthesized dynamic #5#0 = z;
     if(!(#5#0 is{ForNonNullableByDefault} (core::Object?, {quux: core::Object?}) && (let final dynamic #t17 = qux = #5#0{(core::Object?, {quux: core::Object?})}.$1{core::Object?} in true) && (let final dynamic #t18 = quux = #5#0{(core::Object?, {quux: core::Object?})}.quux{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#5#0;
 }
diff --git a/pkg/front_end/testcases/patterns/variable_names.dart.weak.modular.expect b/pkg/front_end/testcases/patterns/variable_names.dart.weak.modular.expect
index 5ee5b071bd5..8d214604f3a 100644
--- a/pkg/front_end/testcases/patterns/variable_names.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/patterns/variable_names.dart.weak.modular.expect
@@ -12,7 +12,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 abstract class A extends core::Object {
   synthetic constructor •() → self::A
@@ -74,11 +73,11 @@ static method test(dynamic x, dynamic y, dynamic z) → dynamic {
   block {
     final synthesized dynamic #4#0 = y;
     if(!(#4#0 is{ForNonNullableByDefault} self::A && (let final dynamic #t15 = qux = #4#0{self::A}.{self::A::qux}{core::bool} in true) && (let final dynamic #t16 = quux = #4#0{self::A}.{self::A::quux}{core::String} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#4#0;
   block {
     final synthesized dynamic #5#0 = z;
     if(!(#5#0 is{ForNonNullableByDefault} (core::Object?, {quux: core::Object?}) && (let final dynamic #t17 = qux = #5#0{(core::Object?, {quux: core::Object?})}.$1{core::Object?} in true) && (let final dynamic #t18 = quux = #5#0{(core::Object?, {quux: core::Object?})}.quux{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#5#0;
 }
diff --git a/pkg/front_end/testcases/patterns/variable_names.dart.weak.transformed.expect b/pkg/front_end/testcases/patterns/variable_names.dart.weak.transformed.expect
index 7c31ba40fda..9f94ace94c8 100644
--- a/pkg/front_end/testcases/patterns/variable_names.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/patterns/variable_names.dart.weak.transformed.expect
@@ -12,7 +12,6 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_internal" as _in;
 
 abstract class A extends core::Object {
   synthetic constructor •() → self::A
@@ -86,11 +85,11 @@ static method test(dynamic x, dynamic y, dynamic z) → dynamic {
   block {
     final synthesized dynamic #4#0 = y;
     if(!(#4#0 is{ForNonNullableByDefault} self::A && (let final core::bool #t15 = qux = #4#0{self::A}.{self::A::qux}{core::bool} in true) && (let final core::String #t16 = quux = #4#0{self::A}.{self::A::quux}{core::String} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#4#0;
   block {
     final synthesized dynamic #5#0 = z;
     if(!(#5#0 is{ForNonNullableByDefault} (core::Object?, {quux: core::Object?}) && (let final core::Object? #t17 = qux = #5#0{(core::Object?, {quux: core::Object?})}.$1{core::Object?} in true) && (let final core::Object? #t18 = quux = #5#0{(core::Object?, {quux: core::Object?})}.quux{core::Object?} in true)))
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#5#0;
 }
diff --git a/pkg/front_end/testcases/rasta/malformed_function.dart.strong.expect b/pkg/front_end/testcases/rasta/malformed_function.dart.strong.expect
index ffb55db05cc..fb7089c5be6 100644
--- a/pkg/front_end/testcases/rasta/malformed_function.dart.strong.expect
+++ b/pkg/front_end/testcases/rasta/malformed_function.dart.strong.expect
@@ -8,7 +8,7 @@ library /*isNonNullableByDefault*/;
 //    ^
 //
 import self as self;
-import "dart:_internal" as _in;
+import "dart:core" as core;
 
 static method main() → dynamic {
   block {
@@ -17,6 +17,6 @@ static method main() → dynamic {
 Try using an if-case, a 'switch' statement, or a 'switch' expression instead.
   (null) = null;
    ^")
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
 }
diff --git a/pkg/front_end/testcases/rasta/malformed_function.dart.strong.transformed.expect b/pkg/front_end/testcases/rasta/malformed_function.dart.strong.transformed.expect
index ffb55db05cc..fb7089c5be6 100644
--- a/pkg/front_end/testcases/rasta/malformed_function.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/rasta/malformed_function.dart.strong.transformed.expect
@@ -8,7 +8,7 @@ library /*isNonNullableByDefault*/;
 //    ^
 //
 import self as self;
-import "dart:_internal" as _in;
+import "dart:core" as core;
 
 static method main() → dynamic {
   block {
@@ -17,6 +17,6 @@ static method main() → dynamic {
 Try using an if-case, a 'switch' statement, or a 'switch' expression instead.
   (null) = null;
    ^")
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
 }
diff --git a/pkg/front_end/testcases/rasta/malformed_function.dart.weak.expect b/pkg/front_end/testcases/rasta/malformed_function.dart.weak.expect
index ffb55db05cc..fb7089c5be6 100644
--- a/pkg/front_end/testcases/rasta/malformed_function.dart.weak.expect
+++ b/pkg/front_end/testcases/rasta/malformed_function.dart.weak.expect
@@ -8,7 +8,7 @@ library /*isNonNullableByDefault*/;
 //    ^
 //
 import self as self;
-import "dart:_internal" as _in;
+import "dart:core" as core;
 
 static method main() → dynamic {
   block {
@@ -17,6 +17,6 @@ static method main() → dynamic {
 Try using an if-case, a 'switch' statement, or a 'switch' expression instead.
   (null) = null;
    ^")
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
 }
diff --git a/pkg/front_end/testcases/rasta/malformed_function.dart.weak.modular.expect b/pkg/front_end/testcases/rasta/malformed_function.dart.weak.modular.expect
index ffb55db05cc..fb7089c5be6 100644
--- a/pkg/front_end/testcases/rasta/malformed_function.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/rasta/malformed_function.dart.weak.modular.expect
@@ -8,7 +8,7 @@ library /*isNonNullableByDefault*/;
 //    ^
 //
 import self as self;
-import "dart:_internal" as _in;
+import "dart:core" as core;
 
 static method main() → dynamic {
   block {
@@ -17,6 +17,6 @@ static method main() → dynamic {
 Try using an if-case, a 'switch' statement, or a 'switch' expression instead.
   (null) = null;
    ^")
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
 }
diff --git a/pkg/front_end/testcases/rasta/malformed_function.dart.weak.transformed.expect b/pkg/front_end/testcases/rasta/malformed_function.dart.weak.transformed.expect
index ffb55db05cc..fb7089c5be6 100644
--- a/pkg/front_end/testcases/rasta/malformed_function.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/rasta/malformed_function.dart.weak.transformed.expect
@@ -8,7 +8,7 @@ library /*isNonNullableByDefault*/;
 //    ^
 //
 import self as self;
-import "dart:_internal" as _in;
+import "dart:core" as core;
 
 static method main() → dynamic {
   block {
@@ -17,6 +17,6 @@ static method main() → dynamic {
 Try using an if-case, a 'switch' statement, or a 'switch' expression instead.
   (null) = null;
    ^")
-      throw new _in::ReachabilityError::•();
+      throw new core::StateError::•("Pattern matching error");
   } =>#0#0;
 }