[kernel] Add isWildcard flag to VariableDeclaration.

Adds the `isWildcard` flag to variables. Will be using this for producing better errors.

TEST= Existing expectations tests for wildcards pass with new flag.
Bug: https://github.com/dart-lang/sdk/issues/55655
Change-Id: If2f7a5555e7cc26e84d1b1e63e4261c81a157d78
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/370062
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

@@ -3842,7 +3842,8 @@ class BodyBuilder extends StackListenerImpl
         isLate: isLate,
         isRequired: isRequired,
         hasDeclaredInitializer: initializer != null,
-        isStaticLate: isFinal && initializer == null)
+        isStaticLate: isFinal && initializer == null,
+        isWildcard: identifier.name == '_')
       ..fileOffset = identifier.nameOffset
       ..fileEqualsOffset = offsetForToken(equalsToken);
     typeInferrer.assignedVariables.declare(variable);
@@ -3900,8 +3901,9 @@ class BodyBuilder extends StackListenerImpl
     push(variable);
 
     // Avoid adding the local identifier to scope if it's a wildcard.
-    if (!(libraryFeatures.wildcardVariables.isEnabled &&
-        variable.name == '_')) {
+    // TODO(kallentu): Emit better error on lookup, rather than not adding it to
+    // the scope.
+    if (!(libraryFeatures.wildcardVariables.isEnabled && variable.isWildcard)) {
       declareVariable(variable, scope);
     }
   }

pkg/front_end/lib/src/fasta/kernel/internal_ast.dart

@@ -1151,7 +1151,8 @@ class VariableDeclarationImpl extends VariableDeclaration {
       bool isRequired = false,
       bool isLowered = false,
       bool isSynthesized = false,
-      this.isStaticLate = false})
+      this.isStaticLate = false,
+      bool isWildcard = false})
       : isImplicitlyTyped = type == null,
         isLocalFunction = isLocalFunction,
         super(name,
@@ -1165,7 +1166,8 @@ class VariableDeclarationImpl extends VariableDeclaration {
             isRequired: isRequired,
             isLowered: isLowered,
             isSynthesized: isSynthesized,
-            hasDeclaredInitializer: hasDeclaredInitializer);
+            hasDeclaredInitializer: hasDeclaredInitializer,
+            isWildcard: isWildcard);
 
   VariableDeclarationImpl.forEffect(Expression initializer)
       : forSyntheticToken = false,

pkg/front_end/testcases/general/arithmetic.dart.strong.expect

@@ -7,7 +7,7 @@ static method foo(core::int x, core::int y) → core::int {
   return z.{core::int::<<}(4){(core::int) → core::int};
 }
 static method loop(core::List<dynamic> xs) → void {
-  core::int _ = xs.{core::List::length}{core::int};
+  wildcard core::int _ = xs.{core::List::length}{core::int};
   for (core::int i = 0; i.{core::num::<}(xs.{core::List::length}{core::int}){(core::num) → core::bool}; i = i.{core::num::+}(1){(core::num) → core::int}) {
   }
 }

pkg/front_end/testcases/general/arithmetic.dart.strong.modular.expect

@@ -7,7 +7,7 @@ static method foo(core::int x, core::int y) → core::int {
   return z.{core::int::<<}(4){(core::int) → core::int};
 }
 static method loop(core::List<dynamic> xs) → void {
-  core::int _ = xs.{core::List::length}{core::int};
+  wildcard core::int _ = xs.{core::List::length}{core::int};
   for (core::int i = 0; i.{core::num::<}(xs.{core::List::length}{core::int}){(core::num) → core::bool}; i = i.{core::num::+}(1){(core::num) → core::int}) {
   }
 }

pkg/front_end/testcases/general/arithmetic.dart.strong.transformed.expect

@@ -7,7 +7,7 @@ static method foo(core::int x, core::int y) → core::int {
   return z.{core::int::<<}(4){(core::int) → core::int};
 }
 static method loop(core::List<dynamic> xs) → void {
-  core::int _ = xs.{core::List::length}{core::int};
+  wildcard core::int _ = xs.{core::List::length}{core::int};
   for (core::int i = 0; i.{core::num::<}(xs.{core::List::length}{core::int}){(core::num) → core::bool}; i = i.{core::num::+}(1){(core::num) → core::int}) {
   }
 }

pkg/front_end/testcases/general/closure.dart.strong.expect

@@ -14,7 +14,7 @@ class Bar extends core::Object {
     ;
 }
 static method useCallback(dynamic callback) → dynamic {
-  dynamic _ = callback{dynamic}();
+  wildcard dynamic _ = callback{dynamic}();
 }
 static method main() → dynamic {
   dynamic x;
@@ -23,5 +23,5 @@ static method main() → dynamic {
     return new self::Foo::•();
   }
   self::useCallback(inner);
-  self::Bar _ = inner(){() → self::Foo}.{self::Foo::_field}{self::Bar};
+  wildcard self::Bar _ = inner(){() → self::Foo}.{self::Foo::_field}{self::Bar};
 }

pkg/front_end/testcases/general/closure.dart.strong.modular.expect

@@ -14,7 +14,7 @@ class Bar extends core::Object {
     ;
 }
 static method useCallback(dynamic callback) → dynamic {
-  dynamic _ = callback{dynamic}();
+  wildcard dynamic _ = callback{dynamic}();
 }
 static method main() → dynamic {
   dynamic x;
@@ -23,5 +23,5 @@ static method main() → dynamic {
     return new self::Foo::•();
   }
   self::useCallback(inner);
-  self::Bar _ = inner(){() → self::Foo}.{self::Foo::_field}{self::Bar};
+  wildcard self::Bar _ = inner(){() → self::Foo}.{self::Foo::_field}{self::Bar};
 }

pkg/front_end/testcases/general/closure.dart.strong.transformed.expect

@@ -14,7 +14,7 @@ class Bar extends core::Object {
     ;
 }
 static method useCallback(dynamic callback) → dynamic {
-  dynamic _ = callback{dynamic}();
+  wildcard dynamic _ = callback{dynamic}();
 }
 static method main() → dynamic {
   dynamic x;
@@ -23,5 +23,5 @@ static method main() → dynamic {
     return new self::Foo::•();
   }
   self::useCallback(inner);
-  self::Bar _ = inner(){() → self::Foo}.{self::Foo::_field}{self::Bar};
+  wildcard self::Bar _ = inner(){() → self::Foo}.{self::Foo::_field}{self::Bar};
 }

pkg/front_end/testcases/general/escape.dart.strong.expect

@@ -28,10 +28,10 @@ class X extends core::Object implements self::A, self::B {
     ;
 }
 static method useAsA(self::A object) → void {
-  dynamic _ = object.{self::A::field}{dynamic};
+  wildcard dynamic _ = object.{self::A::field}{dynamic};
 }
 static method useAsB(self::B object) → void {
-  dynamic _ = object.{self::B::field}{dynamic};
+  wildcard dynamic _ = object.{self::B::field}{dynamic};
   self::escape(object);
 }
 static method escape(dynamic x) → void {

pkg/front_end/testcases/general/escape.dart.strong.modular.expect

@@ -28,10 +28,10 @@ class X extends core::Object implements self::A, self::B {
     ;
 }
 static method useAsA(self::A object) → void {
-  dynamic _ = object.{self::A::field}{dynamic};
+  wildcard dynamic _ = object.{self::A::field}{dynamic};
 }
 static method useAsB(self::B object) → void {
-  dynamic _ = object.{self::B::field}{dynamic};
+  wildcard dynamic _ = object.{self::B::field}{dynamic};
   self::escape(object);
 }
 static method escape(dynamic x) → void {

pkg/front_end/testcases/general/escape.dart.strong.transformed.expect

@@ -28,10 +28,10 @@ class X extends core::Object implements self::A, self::B {
     ;
 }
 static method useAsA(self::A object) → void {
-  dynamic _ = object.{self::A::field}{dynamic};
+  wildcard dynamic _ = object.{self::A::field}{dynamic};
 }
 static method useAsB(self::B object) → void {
-  dynamic _ = object.{self::B::field}{dynamic};
+  wildcard dynamic _ = object.{self::B::field}{dynamic};
   self::escape(object);
 }
 static method escape(dynamic x) → void {

pkg/front_end/testcases/general/issue48347.dart.strong.expect

@@ -5,7 +5,7 @@ import "dart:async" as asy;
 import "dart:async";
 
 static method test(asy::StreamController<void> _eventStreamController) → dynamic async /* emittedValueType= dynamic */ {
-  await for (final void _ in _eventStreamController.{asy::StreamController::stream}{asy::Stream<void>}) {
+  await for (final wildcard void _ in _eventStreamController.{asy::StreamController::stream}{asy::Stream<void>}) {
   }
 }
 static method main() → dynamic {}

pkg/front_end/testcases/general/issue48347.dart.strong.modular.expect

@@ -5,7 +5,7 @@ import "dart:async" as asy;
 import "dart:async";
 
 static method test(asy::StreamController<void> _eventStreamController) → dynamic async /* emittedValueType= dynamic */ {
-  await for (final void _ in _eventStreamController.{asy::StreamController::stream}{asy::Stream<void>}) {
+  await for (final wildcard void _ in _eventStreamController.{asy::StreamController::stream}{asy::Stream<void>}) {
   }
 }
 static method main() → dynamic {}

pkg/front_end/testcases/general/issue48347.dart.strong.transformed.expect

@@ -11,7 +11,7 @@ static method test(asy::StreamController<void> _eventStreamController) → dynam
     synthesized asy::_StreamIterator<void>? :for-iterator = new asy::_StreamIterator::•<void>(:stream);
     try
       while (let dynamic #t1 = asy::_asyncStarMoveNextHelper(:stream) in await :for-iterator.{asy::_StreamIterator::moveNext}(){() → asy::Future<core::bool>}) {
-        final void _ = :for-iterator.{asy::_StreamIterator::current}{void};
+        final wildcard void _ = :for-iterator.{asy::_StreamIterator::current}{void};
         {}
       }
     finally

pkg/front_end/testcases/patterns/issue52345.dart.strong.expect

@@ -6,7 +6,7 @@ static method main() → void {
   final core::List<core::String> works = <core::String>["b", "l"];
   final core::List<core::String> fails = <core::String>["c"];
   for (final core::List<core::String> test in <core::List<core::String>>[works, fails]) {
-    final void _ = block {
+    final wildcard void _ = block {
       void #t1;
       final synthesized core::List<core::String> #0#0 = test;
       synthesized core::int #0#1;

pkg/front_end/testcases/patterns/issue52345.dart.strong.modular.expect

@@ -6,7 +6,7 @@ static method main() → void {
   final core::List<core::String> works = <core::String>["b", "l"];
   final core::List<core::String> fails = <core::String>["c"];
   for (final core::List<core::String> test in <core::List<core::String>>[works, fails]) {
-    final void _ = block {
+    final wildcard void _ = block {
       void #t1;
       final synthesized core::List<core::String> #0#0 = test;
       synthesized core::int #0#1;

pkg/front_end/testcases/patterns/issue52345.dart.strong.transformed.expect

@@ -10,7 +10,7 @@ static method main() → void {
     for (; :sync-for-iterator.{core::Iterator::moveNext}(){() → core::bool}; ) {
       final core::List<core::String> test = :sync-for-iterator.{core::Iterator::current}{core::List<core::String>};
       {
-        final void _ = block {
+        final wildcard void _ = block {
           void #t1;
           final synthesized core::List<core::String> #0#0 = test;
           synthesized core::int #0#1;

pkg/front_end/testcases/records/block_combine_statements.dart.strong.expect

@@ -4,7 +4,7 @@ import "dart:core" as core;
 import "dart:_internal" as _in;
 
 static method main() → void {
-  for ((core::Iterable<core::int>, core::Iterable<core::int>) _ in self::split<core::int>(<core::int>[1, 2, 3])) {
+  for (wildcard(core::Iterable<core::int>, core::Iterable<core::int>) _ in self::split<core::int>(<core::int>[1, 2, 3])) {
   }
 }
 static method split<A extends core::Object? = dynamic>(core::Iterable<self::split::A%> it) → core::Iterable<(core::Iterable<self::split::A%>, core::Iterable<self::split::A%>)>

pkg/front_end/testcases/records/block_combine_statements.dart.strong.modular.expect

@@ -4,7 +4,7 @@ import "dart:core" as core;
 import "dart:_internal" as _in;
 
 static method main() → void {
-  for ((core::Iterable<core::int>, core::Iterable<core::int>) _ in self::split<core::int>(<core::int>[1, 2, 3])) {
+  for (wildcard(core::Iterable<core::int>, core::Iterable<core::int>) _ in self::split<core::int>(<core::int>[1, 2, 3])) {
   }
 }
 static method split<A extends core::Object? = dynamic>(core::Iterable<self::split::A%> it) → core::Iterable<(core::Iterable<self::split::A%>, core::Iterable<self::split::A%>)>

pkg/front_end/testcases/records/block_combine_statements.dart.strong.transformed.expect

@@ -7,7 +7,7 @@ static method main() → void {
   {
     synthesized core::Iterator<(core::Iterable<core::int>, core::Iterable<core::int>)> :sync-for-iterator = self::split<core::int>(core::_GrowableList::_literal3<core::int>(1, 2, 3)).{core::Iterable::iterator}{core::Iterator<(core::Iterable<core::int>, core::Iterable<core::int>)>};
     for (; :sync-for-iterator.{core::Iterator::moveNext}(){() → core::bool}; ) {
-      (core::Iterable<core::int>, core::Iterable<core::int>) _ = :sync-for-iterator.{core::Iterator::current}{(core::Iterable<core::int>, core::Iterable<core::int>)};
+      wildcard(core::Iterable<core::int>, core::Iterable<core::int>) _ = :sync-for-iterator.{core::Iterator::current}{(core::Iterable<core::int>, core::Iterable<core::int>)};
       {}
     }
   }

pkg/front_end/testcases/wildcard_variables/for_loop.dart.strong.expect

@@ -19,19 +19,19 @@ import "dart:core" as core;
 import "dart:async" as asy;
 
 static method main() → dynamic async /* emittedValueType= dynamic */ {
-  for (core::int _ = 0, core::int _ = 2; ; ) {
+  for (wildcard core::int _ = 0, wildcard core::int _ = 2; ; ) {
     core::print(invalid-expression "pkg/front_end/testcases/wildcard_variables/for_loop.dart:8:11: Error: Undefined name '_'.
     print(_);
           ^");
   }
   core::List<dynamic> list = <dynamic>[];
-  for (dynamic _ in list) {
+  for (wildcard dynamic _ in list) {
     core::print(invalid-expression "pkg/front_end/testcases/wildcard_variables/for_loop.dart:13:11: Error: Undefined name '_'.
     print(_);
           ^");
   }
   asy::Stream<dynamic> stream = new asy::_EmptyStream::•<dynamic>();
-  await for (dynamic _ in stream) {
+  await for (wildcard dynamic _ in stream) {
     core::print(invalid-expression "pkg/front_end/testcases/wildcard_variables/for_loop.dart:18:11: Error: Undefined name '_'.
     print(_);
           ^");

pkg/front_end/testcases/wildcard_variables/for_loop.dart.strong.modular.expect

@@ -19,19 +19,19 @@ import "dart:core" as core;
 import "dart:async" as asy;
 
 static method main() → dynamic async /* emittedValueType= dynamic */ {
-  for (core::int _ = 0, core::int _ = 2; ; ) {
+  for (wildcard core::int _ = 0, wildcard core::int _ = 2; ; ) {
     core::print(invalid-expression "pkg/front_end/testcases/wildcard_variables/for_loop.dart:8:11: Error: Undefined name '_'.
     print(_);
           ^");
   }
   core::List<dynamic> list = <dynamic>[];
-  for (dynamic _ in list) {
+  for (wildcard dynamic _ in list) {
     core::print(invalid-expression "pkg/front_end/testcases/wildcard_variables/for_loop.dart:13:11: Error: Undefined name '_'.
     print(_);
           ^");
   }
   asy::Stream<dynamic> stream = new asy::_EmptyStream::•<dynamic>();
-  await for (dynamic _ in stream) {
+  await for (wildcard dynamic _ in stream) {
     core::print(invalid-expression "pkg/front_end/testcases/wildcard_variables/for_loop.dart:18:11: Error: Undefined name '_'.
     print(_);
           ^");

pkg/front_end/testcases/wildcard_variables/for_loop.dart.strong.transformed.expect

@@ -19,7 +19,7 @@ import "dart:core" as core;
 import "dart:async" as asy;
 
 static method main() → dynamic async /* emittedValueType= dynamic */ {
-  for (core::int _ = 0, core::int _ = 2; ; ) {
+  for (wildcard core::int _ = 0, wildcard core::int _ = 2; ; ) {
     core::print(invalid-expression "pkg/front_end/testcases/wildcard_variables/for_loop.dart:8:11: Error: Undefined name '_'.
     print(_);
           ^");
@@ -28,7 +28,7 @@ static method main() → dynamic async /* emittedValueType= dynamic */ {
   {
     synthesized core::Iterator<dynamic> :sync-for-iterator = list.{core::Iterable::iterator}{core::Iterator<dynamic>};
     for (; :sync-for-iterator.{core::Iterator::moveNext}(){() → core::bool}; ) {
-      dynamic _ = :sync-for-iterator.{core::Iterator::current}{dynamic};
+      wildcard dynamic _ = :sync-for-iterator.{core::Iterator::current}{dynamic};
       {
         core::print(invalid-expression "pkg/front_end/testcases/wildcard_variables/for_loop.dart:13:11: Error: Undefined name '_'.
     print(_);
@@ -42,7 +42,7 @@ static method main() → dynamic async /* emittedValueType= dynamic */ {
     synthesized asy::_StreamIterator<dynamic>? :for-iterator = new asy::_StreamIterator::•<dynamic>(:stream);
     try
       while (let dynamic #t1 = asy::_asyncStarMoveNextHelper(:stream) in await :for-iterator.{asy::_StreamIterator::moveNext}(){() → asy::Future<core::bool>}) {
-        dynamic _ = :for-iterator.{asy::_StreamIterator::current}{dynamic};
+        wildcard dynamic _ = :for-iterator.{asy::_StreamIterator::current}{dynamic};
         {
           core::print(invalid-expression "pkg/front_end/testcases/wildcard_variables/for_loop.dart:18:11: Error: Undefined name '_'.
     print(_);

pkg/front_end/testcases/wildcard_variables/local_member_no_shadowing.dart.strong.expect

@@ -8,8 +8,8 @@ class Field extends core::Object {
     : super core::Object::•()
     ;
   method member() → void {
-    core::int _ = 1;
-    core::int _ = 2;
+    wildcard core::int _ = 1;
+    wildcard core::int _ = 2;
     this.{self::Field::_} = 3;
   }
 }

pkg/front_end/testcases/wildcard_variables/local_member_no_shadowing.dart.strong.modular.expect

@@ -8,8 +8,8 @@ class Field extends core::Object {
     : super core::Object::•()
     ;
   method member() → void {
-    core::int _ = 1;
-    core::int _ = 2;
+    wildcard core::int _ = 1;
+    wildcard core::int _ = 2;
     this.{self::Field::_} = 3;
   }
 }

pkg/front_end/testcases/wildcard_variables/local_member_no_shadowing.dart.strong.transformed.expect

@@ -8,8 +8,8 @@ class Field extends core::Object {
     : super core::Object::•()
     ;
   method member() → void {
-    core::int _ = 1;
-    core::int _ = 2;
+    wildcard core::int _ = 1;
+    wildcard core::int _ = 2;
     this.{self::Field::_} = 3;
   }
 }

pkg/front_end/testcases/wildcard_variables/local_var.dart.strong.expect

@@ -10,8 +10,8 @@ import self as self;
 import "dart:core" as core;
 
 static method main() → dynamic {
-  core::int _ = 1;
-  core::int _ = 2;
+  wildcard core::int _ = 1;
+  wildcard core::int _ = 2;
   invalid-expression "pkg/front_end/testcases/wildcard_variables/local_var.dart:8:3: Error: Setter not found: '_'.
   _ = 3;
   ^";

pkg/front_end/testcases/wildcard_variables/local_var.dart.strong.modular.expect

@@ -10,8 +10,8 @@ import self as self;
 import "dart:core" as core;
 
 static method main() → dynamic {
-  core::int _ = 1;
-  core::int _ = 2;
+  wildcard core::int _ = 1;
+  wildcard core::int _ = 2;
   invalid-expression "pkg/front_end/testcases/wildcard_variables/local_var.dart:8:3: Error: Setter not found: '_'.
   _ = 3;
   ^";

pkg/front_end/testcases/wildcard_variables/local_var.dart.strong.transformed.expect

@@ -10,8 +10,8 @@ import self as self;
 import "dart:core" as core;
 
 static method main() → dynamic {
-  core::int _ = 1;
-  core::int _ = 2;
+  wildcard core::int _ = 1;
+  wildcard core::int _ = 2;
   invalid-expression "pkg/front_end/testcases/wildcard_variables/local_var.dart:8:3: Error: Setter not found: '_'.
   _ = 3;
   ^";

pkg/front_end/testcases/wildcard_variables/local_var_no_shadowing.dart.strong.expect

@@ -4,7 +4,7 @@ import "dart:core" as core;
 
 static field core::int _ = 100;
 static method main() → dynamic {
-  core::int _ = 1;
-  core::int _ = 2;
+  wildcard core::int _ = 1;
+  wildcard core::int _ = 2;
   self::_ = 3;
 }

pkg/front_end/testcases/wildcard_variables/local_var_no_shadowing.dart.strong.modular.expect

@@ -4,7 +4,7 @@ import "dart:core" as core;
 
 static field core::int _ = 100;
 static method main() → dynamic {
-  core::int _ = 1;
-  core::int _ = 2;
+  wildcard core::int _ = 1;
+  wildcard core::int _ = 2;
   self::_ = 3;
 }

pkg/front_end/testcases/wildcard_variables/local_var_no_shadowing.dart.strong.transformed.expect

@@ -4,7 +4,7 @@ import "dart:core" as core;
 
 static field core::int _ = 100;
 static method main() → dynamic {
-  core::int _ = 1;
-  core::int _ = 2;
+  wildcard core::int _ = 1;
+  wildcard core::int _ = 2;
   self::_ = 3;
 }

pkg/kernel/binary.md

@@ -147,7 +147,7 @@ type CanonicalName {
 
 type ComponentFile {
   UInt32 magic = 0x90ABCDEF;
-  UInt32 formatVersion = 118;
+  UInt32 formatVersion = 119;
   Byte[10] shortSdkHash;
   List<String> problemsAsJson; // Described in problems.md.
   Library[] libraries;
@@ -1501,7 +1501,7 @@ type VariableDeclarationPlain {
 
   UInt flags (isFinal, isConst, hasDeclaredInitializer, isInitializingFormal,
               isCovariantByClass, isLate, isRequired, isCovariantByDeclaration,
-              isLowered, isSynthesized, isHoisted);
+              isLowered, isSynthesized, isHoisted, isWildcard);
   // For named parameters, this is the parameter name.
   // For other variables, the name is cosmetic, may be empty,
   // and is not necessarily unique.

pkg/kernel/lib/ast.dart

@@ -10513,7 +10513,8 @@ class VariableDeclaration extends Statement implements Annotatable {
       bool isLowered = false,
       bool isSynthesized = false,
       bool isHoisted = false,
-      bool hasDeclaredInitializer = false}) {
+      bool hasDeclaredInitializer = false,
+      bool isWildcard = false}) {
     initializer?.parent = this;
     if (flags != -1) {
       this.flags = flags;
@@ -10528,6 +10529,7 @@ class VariableDeclaration extends Statement implements Annotatable {
       this.hasDeclaredInitializer = hasDeclaredInitializer;
       this.isSynthesized = isSynthesized;
       this.isHoisted = isHoisted;
+      this.isWildcard = isWildcard;
     }
     assert(_name != null || this.isSynthesized,
         "Only synthesized variables can have no name.");
@@ -10576,6 +10578,7 @@ class VariableDeclaration extends Statement implements Annotatable {
   static const int FlagLowered = 1 << 8;
   static const int FlagSynthesized = 1 << 9;
   static const int FlagHoisted = 1 << 10;
+  static const int FlagWildcard = 1 << 11;
 
   bool get isFinal => flags & FlagFinal != 0;
   bool get isConst => flags & FlagConst != 0;
@@ -10641,6 +10644,11 @@ class VariableDeclaration extends Statement implements Annotatable {
   /// as the initializer of the second variable.
   bool get hasDeclaredInitializer => flags & FlagHasDeclaredInitializer != 0;
 
+  /// Whether this variable is a wildcard variable.
+  ///
+  /// Wildcard variables have the name `_`.
+  bool get isWildcard => flags & FlagWildcard != 0;
+
   /// Whether the variable is assignable.
   ///
   /// This is `true` if the variable is neither constant nor final, or if it
@@ -10709,6 +10717,12 @@ class VariableDeclaration extends Statement implements Annotatable {
         : (flags & ~FlagHasDeclaredInitializer);
   }
 
+  void set isWildcard(bool value) {
+    // TODO(kallentu): Change the name to be unique with other wildcard
+    // variables.
+    flags = value ? (flags | FlagWildcard) : (flags & ~FlagWildcard);
+  }
+
   void clearAnnotations() {
     annotations = const <Expression>[];
   }

pkg/kernel/lib/binary/tag.dart

@@ -226,7 +226,7 @@ class Tag {
   /// Internal version of kernel binary format.
   /// Bump it when making incompatible changes in kernel binaries.
   /// Keep in sync with runtime/vm/kernel_binary.h, pkg/kernel/binary.md.
-  static const int BinaryFormatVersion = 118;
+  static const int BinaryFormatVersion = 119;
 }
 
 abstract class ConstantTag {

pkg/kernel/lib/text/ast_to_text.dart

@@ -2452,6 +2452,7 @@ class Printer extends VisitorDefault<void> with VisitorVoidMixin {
     writeModifier(node.isConst, 'const');
     writeModifier(node.isSynthesized && node.name != null, 'synthesized');
     writeModifier(node.isHoisted, 'hoisted');
+    writeModifier(node.isWildcard, 'wildcard');
     bool hasImplicitInitializer = node.initializer is NullLiteral ||
         (node.initializer is ConstantExpression &&
             (node.initializer as ConstantExpression).constant is NullConstant);

runtime/vm/compiler/frontend/kernel_translation_helper.h

@@ -419,6 +419,7 @@ class VariableDeclarationHelper {
     kLowered = 1 << 8,
     kSynthesized = 1 << 9,
     kHoisted = 1 << 10,
+    kWildcard = 1 << 11,
   };
 
   explicit VariableDeclarationHelper(KernelReaderHelper* helper)
@@ -440,6 +441,7 @@ class VariableDeclarationHelper {
   bool IsRequired() const { return (flags_ & kRequired) != 0; }
   bool IsSynthesized() const { return (flags_ & kSynthesized) != 0; }
   bool IsHoisted() const { return (flags_ & kHoisted) != 0; }
+  bool IsWildcard() const { return (flags_ & kWildcard) != 0; }
   bool HasDeclaredInitializer() const {
     return (flags_ & kHasDeclaredInitializer) != 0;
   }

runtime/vm/kernel_binary.h

@@ -18,7 +18,7 @@ namespace kernel {
 // package:kernel/binary.md.
 
 static const uint32_t kMagicProgramFile = 0x90ABCDEFu;
-static const uint32_t kSupportedKernelFormatVersion = 118;
+static const uint32_t kSupportedKernelFormatVersion = 119;
 
 // Keep in sync with package:kernel/lib/binary/tag.dart
 #define KERNEL_TAG_LIST(V)                                                     \