[cfe] Remove support for old super-mixins feature

This CL remove the support for mixing in classes that don't extend
Object. An error has been report by the analyzer for some time and
now also by the CFE.

This puts the breaking change https://github.com/dart-lang/sdk/issues/48167 into effect.

TEST=pkg/front_end/testcases/general/issue48167.dart

Change-Id: Ia7715a27dc1aa18a7c85b24ed86d19a91b6924d5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/247551
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

CHANGELOG.md

@@ -1,3 +1,23 @@
+## 2.19.0
+
+### Language
+
+- **Breaking Change** [#48167](https://github.com/dart-lang/sdk/issues/48167):
+  Mixin of classes that don't extend `Object` is no longer supported:
+  ```dart
+  class Base {}
+  class Mixin extends Base {}
+  class C extends Base with Mixin {}
+  ```
+  This should instead be written using a mixin declaration of `Mixin`:
+  ```dart
+  class Base {}
+  mixin Mixin on Base {}
+  class C extends Base with Mixin {}
+  ```
+  This feature has not been supported in most compilation targets for some
+  time but is now completely removed.
+
 ## 2.18.0
 
 ### Language

pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart

@@ -7838,6 +7838,31 @@ const MessageCode messageMixinFunction = const MessageCode("MixinFunction",
     problemMessage: r"""Mixing in 'Function' is deprecated.""",
     correctionMessage: r"""Try removing 'Function' from the 'with' clause.""");
 
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Template<
+    Message Function(
+        String
+            name)> templateMixinInheritsFromNotObject = const Template<
+        Message Function(String name)>(
+    problemMessageTemplate:
+        r"""The class '#name' can't be used as a mixin because it extends a class other than 'Object'.""",
+    withArguments: _withArgumentsMixinInheritsFromNotObject);
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Code<Message Function(String name)> codeMixinInheritsFromNotObject =
+    const Code<Message Function(String name)>("MixinInheritsFromNotObject",
+        analyzerCodes: <String>["MIXIN_INHERITS_FROM_NOT_OBJECT"]);
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+Message _withArgumentsMixinInheritsFromNotObject(String name) {
+  if (name.isEmpty) throw 'No name provided';
+  name = demangleMixinApplicationName(name);
+  return new Message(codeMixinInheritsFromNotObject,
+      problemMessage:
+          """The class '${name}' can't be used as a mixin because it extends a class other than 'Object'.""",
+      arguments: {'name': name});
+}
+
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Template<
     Message Function(

pkg/compiler/test/model/supermixin_test.dart

@@ -23,7 +23,7 @@ class SuperB extends SuperA {
   method1(a) => 'B$a';
 }
 
-class Mixin extends SuperA {
+mixin Mixin on SuperA {
   method1(a) => super.method1('M$a');
   method2(a) => 'M$a';
 }
@@ -55,8 +55,10 @@ main() {
 
     Expect.isTrue(elementEnvironment.isMixinApplicationWithMembers(superClass));
     Expect.equals(mixin, elementEnvironment.getEffectiveMixinClass(superClass));
-    Expect.equals(superA, elementEnvironment.getSuperClass(mixin));
     Expect.equals(superB, elementEnvironment.getSuperClass(superClass));
+    Expect.equals(superA, elementEnvironment.getSuperClass(superB));
+    Expect.equals(elementEnvironment.getSuperClass(superA),
+        elementEnvironment.getSuperClass(mixin));
 
     MemberEntity method1 = lookupMember(elementEnvironment, 'Class.method1');
     Expect.equals(superClass, method1.enclosingClass);

pkg/front_end/lib/src/fasta/builder/class_builder.dart

@@ -417,9 +417,7 @@ abstract class ClassBuilderImpl extends DeclarationBuilderImpl
         ? hierarchy.getDispatchTarget(instanceClass, name, setter: isSetter)
         : hierarchy.getInterfaceMember(instanceClass, name, setter: isSetter);
     if (isSuper && target == null) {
-      if (cls.isMixinDeclaration ||
-          (libraryBuilder.loader.target.backendTarget.enableSuperMixins &&
-              this.isAbstract)) {
+      if (cls.isMixinDeclaration) {
         target =
             hierarchy.getInterfaceMember(instanceClass, name, setter: isSetter);
       }

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

@@ -487,8 +487,8 @@ class KernelTarget extends TargetImplementation {
       loader.buildClassHierarchy(sortedSourceClassBuilders, objectClassBuilder);
 
       benchmarker?.enterPhase(BenchmarkPhases.outline_checkSupertypes);
-      loader.checkSupertypes(
-          sortedSourceClassBuilders, enumClass, underscoreEnumClass);
+      loader.checkSupertypes(sortedSourceClassBuilders, objectClass, enumClass,
+          underscoreEnumClass);
 
       if (macroApplications != null) {
         benchmarker?.enterPhase(BenchmarkPhases.outline_applyDeclarationMacros);

pkg/front_end/lib/src/fasta/source/source_class_builder.dart

@@ -41,6 +41,7 @@ import '../dill/dill_member_builder.dart';
 import '../fasta_codes.dart';
 import '../kernel/combined_member_signature.dart';
 import '../kernel/hierarchy/hierarchy_builder.dart';
+import '../kernel/hierarchy/hierarchy_node.dart';
 import '../kernel/kernel_helper.dart';
 import '../kernel/kernel_target.dart' show KernelTarget;
 import '../kernel/redirecting_factory_body.dart'
@@ -729,6 +730,7 @@ class SourceClassBuilder extends ClassBuilderImpl
   void checkSupertypes(
       CoreTypes coreTypes,
       ClassHierarchyBuilder hierarchyBuilder,
+      Class objectClass,
       Class enumClass,
       Class underscoreEnumClass,
       Class? macroClass) {
@@ -738,16 +740,17 @@ class SourceClassBuilder extends ClassBuilderImpl
     // Moreover, it checks that `FutureOr` and `void` are not among the
     // supertypes and that `Enum` is not implemented by non-abstract classes.
 
+    ClassHierarchyNode classHierarchyNode =
+        hierarchyBuilder.getNodeFromClass(cls);
     if (libraryBuilder.libraryFeatures.enhancedEnums.isEnabled && !isEnum) {
       bool hasEnumSuperinterface = false;
-      List<Supertype> interfaces =
-          hierarchyBuilder.getNodeFromClass(cls).superclasses;
+      List<Supertype> interfaces = classHierarchyNode.superclasses;
       for (int i = 0; !hasEnumSuperinterface && i < interfaces.length; i++) {
         if (interfaces[i].classNode == enumClass) {
           hasEnumSuperinterface = true;
         }
       }
-      interfaces = hierarchyBuilder.getNodeFromClass(cls).interfaces;
+      interfaces = classHierarchyNode.interfaces;
       for (int i = 0; !hasEnumSuperinterface && i < interfaces.length; i++) {
         if (interfaces[i].classNode == enumClass) {
           hasEnumSuperinterface = true;
@@ -810,14 +813,13 @@ class SourceClassBuilder extends ClassBuilderImpl
     if (macroClass != null && !cls.isMacro && !cls.isAbstract) {
       // TODO(johnniwinther): Merge this check with the loop above.
       bool isMacroFound = false;
-      List<Supertype> interfaces =
-          hierarchyBuilder.getNodeFromClass(cls).superclasses;
+      List<Supertype> interfaces = classHierarchyNode.superclasses;
       for (int i = 0; !isMacroFound && i < interfaces.length; i++) {
         if (interfaces[i].classNode == macroClass) {
           isMacroFound = true;
         }
       }
-      interfaces = hierarchyBuilder.getNodeFromClass(cls).interfaces;
+      interfaces = classHierarchyNode.interfaces;
       for (int i = 0; !isMacroFound && i < interfaces.length; i++) {
         if (interfaces[i].classNode == macroClass) {
           isMacroFound = true;
@@ -866,6 +868,23 @@ class SourceClassBuilder extends ClassBuilderImpl
         superClass = decl;
       }
     }
+    if (classHierarchyNode.isMixinApplication) {
+      assert(mixedInTypeBuilder != null,
+          "No mixed in type builder for mixin application $this.");
+      ClassHierarchyNode mixedInNode = classHierarchyNode.mixedInNode!;
+      ClassHierarchyNode? mixinSuperClassNode =
+          mixedInNode.directSuperClassNode;
+      if (mixinSuperClassNode != null &&
+          mixinSuperClassNode.classBuilder.cls != objectClass &&
+          !mixedInNode.classBuilder.cls.isMixinDeclaration) {
+        addProblem(
+            templateMixinInheritsFromNotObject
+                .withArguments(mixedInNode.classBuilder.name),
+            mixedInTypeBuilder!.charOffset ?? TreeNode.noOffset,
+            noLength);
+      }
+    }
+
     if (interfaceBuilders == null) return;
 
     // Validate interfaces.

pkg/front_end/lib/src/fasta/source/source_loader.dart

@@ -2117,12 +2117,12 @@ severity: $severity
     ticker.logMs("Computed core types");
   }
 
-  void checkSupertypes(List<SourceClassBuilder> sourceClasses, Class enumClass,
-      Class underscoreEnumClass) {
+  void checkSupertypes(List<SourceClassBuilder> sourceClasses,
+      Class objectClass, Class enumClass, Class underscoreEnumClass) {
     for (SourceClassBuilder builder in sourceClasses) {
       if (builder.libraryBuilder.loader == this && !builder.isPatch) {
-        builder.checkSupertypes(coreTypes, hierarchyBuilder, enumClass,
-            underscoreEnumClass, _macroClassBuilder?.cls);
+        builder.checkSupertypes(coreTypes, hierarchyBuilder, objectClass,
+            enumClass, underscoreEnumClass, _macroClassBuilder?.cls);
       }
     }
     ticker.logMs("Checked supertypes");

pkg/front_end/messages.yaml

@@ -5604,3 +5604,11 @@ EnumImplementerContainsRestrictedInstanceDeclaration:
 
 EnumAbstractMember:
   problemMessage: "Enums can't declare abstract members."
+
+MixinInheritsFromNotObject:
+  problemMessage: "The class '#name' can't be used as a mixin because it extends a class other than 'Object'."
+  analyzerCode: MIXIN_INHERITS_FROM_NOT_OBJECT
+  script: |
+    class Base {}
+    class Mixin extends Base {}
+    class C extends Base with Mixin {}

pkg/front_end/test/fasta/super_mixins_test.dart

@@ -108,7 +108,7 @@ Future<void> testEnabledSuperMixins() async {
     ..target = new NoneTargetWithSuperMixins(new TargetFlags());
   await compileScript(testSource, options: options);
   Expect.setEquals(
-      const <String>['baz', 'foo', 'quux'], missingSuperMethodNames);
+      const <String>['bar', 'baz', 'foo', 'quux'], missingSuperMethodNames);
 }
 
 void main() {

pkg/front_end/test/macros/application/data/tests/sequence.dart

@@ -166,7 +166,6 @@ augment class Class6a {
 */
 class Class6a {}
 
-@SequenceMacro(0)
 /*class: Class7d:
 declarations:
 
@@ -174,7 +173,8 @@ augment class Class7d {
   method2() {}
 }
 */
-class Class7d with Class7b, Class7c {}
+@SequenceMacro(0)
+class Class7d extends Class7b with Class7c {}
 
 @SequenceMacro(0)
 /*class: Class7c:

pkg/front_end/test/macros/application/data/tests/sequence.dart.expect

@@ -95,24 +95,17 @@ class Class6a extends core::Object {
     ;
   method /* from org-dartlang-augmentation:/a/b/c/main.dart-11 */ method() → dynamic {}
 }
-abstract class _Class7d&Object&Class7b = core::Object with self::Class7b /*isAnonymousMixin,hasConstConstructor*/  {
-  const synthetic constructor •() → self::_Class7d&Object&Class7b
-    : super core::Object::•()
-    ;
-  mixin-super-stub method method1() → dynamic
-    return super.{self::Class7b::method1}();
-}
-abstract class _Class7d&Object&Class7b&Class7c = self::_Class7d&Object&Class7b with self::Class7c /*isAnonymousMixin,hasConstConstructor*/  {
-  const synthetic constructor •() → self::_Class7d&Object&Class7b&Class7c
-    : super self::_Class7d&Object&Class7b::•()
+abstract class _Class7d&Class7b&Class7c = self::Class7b with self::Class7c /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Class7d&Class7b&Class7c
+    : super self::Class7b::•()
     ;
   mixin-super-stub method method() → dynamic
     return super.{self::Class7c::method}();
 }
 @#C2
-class Class7d extends self::_Class7d&Object&Class7b&Class7c {
+class Class7d extends self::_Class7d&Class7b&Class7c {
   synthetic constructor •() → self::Class7d
-    : super self::_Class7d&Object&Class7b&Class7c::•()
+    : super self::_Class7d&Class7b&Class7c::•()
     ;
   method /* from org-dartlang-augmentation:/a/b/c/main.dart-17 */ method2() → dynamic {}
 }

pkg/front_end/testcases/enhanced_enums/enum_as_supertype.dart

@@ -6,7 +6,7 @@ abstract class A extends Enum { // Ok.
   int get foo => index;
 }
 
-enum EA with A { element }
+enum EA with A { element } // Error.
 
 abstract class B implements Enum { // Ok.
   int get foo => index;

pkg/front_end/testcases/enhanced_enums/enum_as_supertype.dart.strong.expect

@@ -1,4 +1,11 @@
 library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/enhanced_enums/enum_as_supertype.dart:9:14: Error: The class 'A' can't be used as a mixin because it extends a class other than 'Object'.
+// enum EA with A { element } // Error.
+//              ^
+//
 import self as self;
 import "dart:core" as core;
 

pkg/front_end/testcases/enhanced_enums/enum_as_supertype.dart.strong.transformed.expect

@@ -1,4 +1,11 @@
 library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/enhanced_enums/enum_as_supertype.dart:9:14: Error: The class 'A' can't be used as a mixin because it extends a class other than 'Object'.
+// enum EA with A { element } // Error.
+//              ^
+//
 import self as self;
 import "dart:core" as core;
 

pkg/front_end/testcases/enhanced_enums/enum_as_supertype.dart.weak.expect

@@ -1,4 +1,11 @@
 library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/enhanced_enums/enum_as_supertype.dart:9:14: Error: The class 'A' can't be used as a mixin because it extends a class other than 'Object'.
+// enum EA with A { element } // Error.
+//              ^
+//
 import self as self;
 import "dart:core" as core;
 

pkg/front_end/testcases/enhanced_enums/enum_as_supertype.dart.weak.modular.expect

@@ -1,4 +1,11 @@
 library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/enhanced_enums/enum_as_supertype.dart:9:14: Error: The class 'A' can't be used as a mixin because it extends a class other than 'Object'.
+// enum EA with A { element } // Error.
+//              ^
+//
 import self as self;
 import "dart:core" as core;
 

pkg/front_end/testcases/enhanced_enums/enum_as_supertype.dart.weak.outline.expect

@@ -1,4 +1,11 @@
 library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/enhanced_enums/enum_as_supertype.dart:9:14: Error: The class 'A' can't be used as a mixin because it extends a class other than 'Object'.
+// enum EA with A { element } // Error.
+//              ^
+//
 import self as self;
 import "dart:core" as core;
 

pkg/front_end/testcases/enhanced_enums/enum_as_supertype.dart.weak.transformed.expect

@@ -1,4 +1,11 @@
 library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/enhanced_enums/enum_as_supertype.dart:9:14: Error: The class 'A' can't be used as a mixin because it extends a class other than 'Object'.
+// enum EA with A { element } // Error.
+//              ^
+//
 import self as self;
 import "dart:core" as core;
 

pkg/front_end/testcases/general/issue48167.dart

@@ -0,0 +1,21 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class Base {}
+
+class Mixin1a extends Base {}
+
+class Mixin1b extends Mixin1a {}
+
+class C1a extends Base with Mixin1a {}
+
+class C1b extends Base with Mixin1b {}
+
+mixin Mixin2a on Base {}
+
+mixin Mixin2b on Base, Mixin2a {}
+
+class C2a extends Base with Mixin2a {}
+
+class C2b extends Base with Mixin2a, Mixin2b {}

pkg/front_end/testcases/general/issue48167.dart.textual_outline.expect

@@ -0,0 +1,16 @@
+class Base {}
+
+class Mixin1a extends Base {}
+
+class Mixin1b extends Mixin1a {}
+
+class C1a extends Base with Mixin1a {}
+
+class C1b extends Base with Mixin1b {}
+
+mixin Mixin2a on Base {}
+mixin Mixin2b on Base, Mixin2a {}
+
+class C2a extends Base with Mixin2a {}
+
+class C2b extends Base with Mixin2a, Mixin2b {}

pkg/front_end/testcases/general/issue48167.dart.textual_outline_modelled.expect

@@ -0,0 +1,16 @@
+class Base {}
+
+class C1a extends Base with Mixin1a {}
+
+class C1b extends Base with Mixin1b {}
+
+class C2a extends Base with Mixin2a {}
+
+class C2b extends Base with Mixin2a, Mixin2b {}
+
+class Mixin1a extends Base {}
+
+class Mixin1b extends Mixin1a {}
+
+mixin Mixin2a on Base {}
+mixin Mixin2b on Base, Mixin2a {}

pkg/front_end/testcases/general/issue48167.dart.weak.expect

@@ -0,0 +1,84 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/issue48167.dart:11:29: Error: The class 'Mixin1a' can't be used as a mixin because it extends a class other than 'Object'.
+// class C1a extends Base with Mixin1a {}
+//                             ^
+//
+// pkg/front_end/testcases/general/issue48167.dart:13:29: Error: The class 'Mixin1b' can't be used as a mixin because it extends a class other than 'Object'.
+// class C1b extends Base with Mixin1b {}
+//                             ^
+//
+import self as self;
+import "dart:core" as core;
+
+class Base extends core::Object {
+  synthetic constructor •() → self::Base
+    : super core::Object::•()
+    ;
+}
+class Mixin1a extends self::Base {
+  synthetic constructor •() → self::Mixin1a
+    : super self::Base::•()
+    ;
+}
+class Mixin1b extends self::Mixin1a {
+  synthetic constructor •() → self::Mixin1b
+    : super self::Mixin1a::•()
+    ;
+}
+abstract class _C1a&Base&Mixin1a = self::Base with self::Mixin1a /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C1a&Base&Mixin1a
+    : super self::Base::•()
+    ;
+}
+class C1a extends self::_C1a&Base&Mixin1a {
+  synthetic constructor •() → self::C1a
+    : super self::_C1a&Base&Mixin1a::•()
+    ;
+}
+abstract class _C1b&Base&Mixin1b = self::Base with self::Mixin1b /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C1b&Base&Mixin1b
+    : super self::Base::•()
+    ;
+}
+class C1b extends self::_C1b&Base&Mixin1b {
+  synthetic constructor •() → self::C1b
+    : super self::_C1b&Base&Mixin1b::•()
+    ;
+}
+abstract class Mixin2a extends self::Base /*isMixinDeclaration*/  {
+}
+abstract class _Mixin2b&Base&Mixin2a extends core::Object implements self::Base, self::Mixin2a /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Mixin2b&Base&Mixin2a
+    : super core::Object::•()
+    ;
+}
+abstract class Mixin2b extends self::_Mixin2b&Base&Mixin2a /*isMixinDeclaration*/  {
+}
+abstract class _C2a&Base&Mixin2a = self::Base with self::Mixin2a /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C2a&Base&Mixin2a
+    : super self::Base::•()
+    ;
+}
+class C2a extends self::_C2a&Base&Mixin2a {
+  synthetic constructor •() → self::C2a
+    : super self::_C2a&Base&Mixin2a::•()
+    ;
+}
+abstract class _C2b&Base&Mixin2a = self::Base with self::Mixin2a /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C2b&Base&Mixin2a
+    : super self::Base::•()
+    ;
+}
+abstract class _C2b&Base&Mixin2a&Mixin2b = self::_C2b&Base&Mixin2a with self::Mixin2b /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C2b&Base&Mixin2a&Mixin2b
+    : super self::_C2b&Base&Mixin2a::•()
+    ;
+}
+class C2b extends self::_C2b&Base&Mixin2a&Mixin2b {
+  synthetic constructor •() → self::C2b
+    : super self::_C2b&Base&Mixin2a&Mixin2b::•()
+    ;
+}

pkg/front_end/testcases/general/issue48167.dart.weak.modular.expect

@@ -0,0 +1,84 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/issue48167.dart:11:29: Error: The class 'Mixin1a' can't be used as a mixin because it extends a class other than 'Object'.
+// class C1a extends Base with Mixin1a {}
+//                             ^
+//
+// pkg/front_end/testcases/general/issue48167.dart:13:29: Error: The class 'Mixin1b' can't be used as a mixin because it extends a class other than 'Object'.
+// class C1b extends Base with Mixin1b {}
+//                             ^
+//
+import self as self;
+import "dart:core" as core;
+
+class Base extends core::Object {
+  synthetic constructor •() → self::Base
+    : super core::Object::•()
+    ;
+}
+class Mixin1a extends self::Base {
+  synthetic constructor •() → self::Mixin1a
+    : super self::Base::•()
+    ;
+}
+class Mixin1b extends self::Mixin1a {
+  synthetic constructor •() → self::Mixin1b
+    : super self::Mixin1a::•()
+    ;
+}
+abstract class _C1a&Base&Mixin1a = self::Base with self::Mixin1a /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C1a&Base&Mixin1a
+    : super self::Base::•()
+    ;
+}
+class C1a extends self::_C1a&Base&Mixin1a {
+  synthetic constructor •() → self::C1a
+    : super self::_C1a&Base&Mixin1a::•()
+    ;
+}
+abstract class _C1b&Base&Mixin1b = self::Base with self::Mixin1b /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C1b&Base&Mixin1b
+    : super self::Base::•()
+    ;
+}
+class C1b extends self::_C1b&Base&Mixin1b {
+  synthetic constructor •() → self::C1b
+    : super self::_C1b&Base&Mixin1b::•()
+    ;
+}
+abstract class Mixin2a extends self::Base /*isMixinDeclaration*/  {
+}
+abstract class _Mixin2b&Base&Mixin2a extends core::Object implements self::Base, self::Mixin2a /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Mixin2b&Base&Mixin2a
+    : super core::Object::•()
+    ;
+}
+abstract class Mixin2b extends self::_Mixin2b&Base&Mixin2a /*isMixinDeclaration*/  {
+}
+abstract class _C2a&Base&Mixin2a = self::Base with self::Mixin2a /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C2a&Base&Mixin2a
+    : super self::Base::•()
+    ;
+}
+class C2a extends self::_C2a&Base&Mixin2a {
+  synthetic constructor •() → self::C2a
+    : super self::_C2a&Base&Mixin2a::•()
+    ;
+}
+abstract class _C2b&Base&Mixin2a = self::Base with self::Mixin2a /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C2b&Base&Mixin2a
+    : super self::Base::•()
+    ;
+}
+abstract class _C2b&Base&Mixin2a&Mixin2b = self::_C2b&Base&Mixin2a with self::Mixin2b /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C2b&Base&Mixin2a&Mixin2b
+    : super self::_C2b&Base&Mixin2a::•()
+    ;
+}
+class C2b extends self::_C2b&Base&Mixin2a&Mixin2b {
+  synthetic constructor •() → self::C2b
+    : super self::_C2b&Base&Mixin2a&Mixin2b::•()
+    ;
+}

pkg/front_end/testcases/general/issue48167.dart.weak.outline.expect

@@ -0,0 +1,76 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/issue48167.dart:11:29: Error: The class 'Mixin1a' can't be used as a mixin because it extends a class other than 'Object'.
+// class C1a extends Base with Mixin1a {}
+//                             ^
+//
+// pkg/front_end/testcases/general/issue48167.dart:13:29: Error: The class 'Mixin1b' can't be used as a mixin because it extends a class other than 'Object'.
+// class C1b extends Base with Mixin1b {}
+//                             ^
+//
+import self as self;
+import "dart:core" as core;
+
+class Base extends core::Object {
+  synthetic constructor •() → self::Base
+    ;
+}
+class Mixin1a extends self::Base {
+  synthetic constructor •() → self::Mixin1a
+    ;
+}
+class Mixin1b extends self::Mixin1a {
+  synthetic constructor •() → self::Mixin1b
+    ;
+}
+abstract class _C1a&Base&Mixin1a = self::Base with self::Mixin1a /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C1a&Base&Mixin1a
+    : super self::Base::•()
+    ;
+}
+class C1a extends self::_C1a&Base&Mixin1a {
+  synthetic constructor •() → self::C1a
+    ;
+}
+abstract class _C1b&Base&Mixin1b = self::Base with self::Mixin1b /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C1b&Base&Mixin1b
+    : super self::Base::•()
+    ;
+}
+class C1b extends self::_C1b&Base&Mixin1b {
+  synthetic constructor •() → self::C1b
+    ;
+}
+abstract class Mixin2a extends self::Base /*isMixinDeclaration*/  {
+}
+abstract class _Mixin2b&Base&Mixin2a extends core::Object implements self::Base, self::Mixin2a /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Mixin2b&Base&Mixin2a
+    ;
+}
+abstract class Mixin2b extends self::_Mixin2b&Base&Mixin2a /*isMixinDeclaration*/  {
+}
+abstract class _C2a&Base&Mixin2a = self::Base with self::Mixin2a /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C2a&Base&Mixin2a
+    : super self::Base::•()
+    ;
+}
+class C2a extends self::_C2a&Base&Mixin2a {
+  synthetic constructor •() → self::C2a
+    ;
+}
+abstract class _C2b&Base&Mixin2a = self::Base with self::Mixin2a /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C2b&Base&Mixin2a
+    : super self::Base::•()
+    ;
+}
+abstract class _C2b&Base&Mixin2a&Mixin2b = self::_C2b&Base&Mixin2a with self::Mixin2b /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_C2b&Base&Mixin2a&Mixin2b
+    : super self::_C2b&Base&Mixin2a::•()
+    ;
+}
+class C2b extends self::_C2b&Base&Mixin2a&Mixin2b {
+  synthetic constructor •() → self::C2b
+    ;
+}

pkg/front_end/testcases/general/issue48167.dart.weak.transformed.expect

@@ -0,0 +1,84 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/issue48167.dart:11:29: Error: The class 'Mixin1a' can't be used as a mixin because it extends a class other than 'Object'.
+// class C1a extends Base with Mixin1a {}
+//                             ^
+//
+// pkg/front_end/testcases/general/issue48167.dart:13:29: Error: The class 'Mixin1b' can't be used as a mixin because it extends a class other than 'Object'.
+// class C1b extends Base with Mixin1b {}
+//                             ^
+//
+import self as self;
+import "dart:core" as core;
+
+class Base extends core::Object {
+  synthetic constructor •() → self::Base
+    : super core::Object::•()
+    ;
+}
+class Mixin1a extends self::Base {
+  synthetic constructor •() → self::Mixin1a
+    : super self::Base::•()
+    ;
+}
+class Mixin1b extends self::Mixin1a {
+  synthetic constructor •() → self::Mixin1b
+    : super self::Mixin1a::•()
+    ;
+}
+abstract class _C1a&Base&Mixin1a extends self::Base implements self::Mixin1a /*isAnonymousMixin,isEliminatedMixin*/  {
+  synthetic constructor •() → self::_C1a&Base&Mixin1a
+    : super self::Base::•()
+    ;
+}
+class C1a extends self::_C1a&Base&Mixin1a {
+  synthetic constructor •() → self::C1a
+    : super self::_C1a&Base&Mixin1a::•()
+    ;
+}
+abstract class _C1b&Base&Mixin1b extends self::Base implements self::Mixin1b /*isAnonymousMixin,isEliminatedMixin*/  {
+  synthetic constructor •() → self::_C1b&Base&Mixin1b
+    : super self::Base::•()
+    ;
+}
+class C1b extends self::_C1b&Base&Mixin1b {
+  synthetic constructor •() → self::C1b
+    : super self::_C1b&Base&Mixin1b::•()
+    ;
+}
+abstract class Mixin2a extends self::Base /*isMixinDeclaration*/  {
+}
+abstract class _Mixin2b&Base&Mixin2a extends core::Object implements self::Base, self::Mixin2a /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Mixin2b&Base&Mixin2a
+    : super core::Object::•()
+    ;
+}
+abstract class Mixin2b extends self::_Mixin2b&Base&Mixin2a /*isMixinDeclaration*/  {
+}
+abstract class _C2a&Base&Mixin2a extends self::Base implements self::Mixin2a /*isAnonymousMixin,isEliminatedMixin*/  {
+  synthetic constructor •() → self::_C2a&Base&Mixin2a
+    : super self::Base::•()
+    ;
+}
+class C2a extends self::_C2a&Base&Mixin2a {
+  synthetic constructor •() → self::C2a
+    : super self::_C2a&Base&Mixin2a::•()
+    ;
+}
+abstract class _C2b&Base&Mixin2a extends self::Base implements self::Mixin2a /*isAnonymousMixin,isEliminatedMixin*/  {
+  synthetic constructor •() → self::_C2b&Base&Mixin2a
+    : super self::Base::•()
+    ;
+}
+abstract class _C2b&Base&Mixin2a&Mixin2b extends self::_C2b&Base&Mixin2a implements self::Mixin2b /*isAnonymousMixin,isEliminatedMixin*/  {
+  synthetic constructor •() → self::_C2b&Base&Mixin2a&Mixin2b
+    : super self::_C2b&Base&Mixin2a::•()
+    ;
+}
+class C2b extends self::_C2b&Base&Mixin2a&Mixin2b {
+  synthetic constructor •() → self::C2b
+    : super self::_C2b&Base&Mixin2a&Mixin2b::•()
+    ;
+}

pkg/front_end/testcases/general/mixin_super_repeated.dart

@@ -6,7 +6,7 @@ abstract class M {
   var m;
 }
 
-abstract class N extends M {
+mixin N on M {
   void set superM(value) {
     super.m = value;
   }

pkg/front_end/testcases/general/mixin_super_repeated.dart.textual_outline.expect

@@ -2,7 +2,7 @@ abstract class M {
   var m;
 }
 
-abstract class N extends M {
+mixin N on M {
   void set superM(value) {}
   get superM => super.m;
 }

pkg/front_end/testcases/general/mixin_super_repeated.dart.textual_outline_modelled.expect

@@ -2,13 +2,12 @@ abstract class M {
   var m;
 }
 
-abstract class N extends M {
-  get superM => super.m;
-  void set superM(value) {}
-}
-
 class Named = S with M, N, M;
 
 class S {}
 
 main() {}
+mixin N on M {
+  get superM => super.m;
+  void set superM(value) {}
+}

pkg/front_end/testcases/general/mixin_super_repeated.dart.weak.expect

@@ -8,10 +8,7 @@ abstract class M extends core::Object {
     : super core::Object::•()
     ;
 }
-abstract class N extends self::M {
-  synthetic constructor •() → self::N
-    : super self::M::•()
-    ;
+abstract class N extends self::M /*isMixinDeclaration*/  {
   set superM(dynamic value) → void {
     super.{self::M::m} = value;
   }

pkg/front_end/testcases/general/mixin_super_repeated.dart.weak.modular.expect

@@ -8,10 +8,7 @@ abstract class M extends core::Object {
     : super core::Object::•()
     ;
 }
-abstract class N extends self::M {
-  synthetic constructor •() → self::N
-    : super self::M::•()
-    ;
+abstract class N extends self::M /*isMixinDeclaration*/  {
   set superM(dynamic value) → void {
     super.{self::M::m} = value;
   }

pkg/front_end/testcases/general/mixin_super_repeated.dart.weak.outline.expect

@@ -7,9 +7,7 @@ abstract class M extends core::Object {
   synthetic constructor •() → self::M
     ;
 }
-abstract class N extends self::M {
-  synthetic constructor •() → self::N
-    ;
+abstract class N extends self::M /*isMixinDeclaration*/  {
   set superM(dynamic value) → void
     ;
   get superM() → dynamic

pkg/front_end/testcases/general/mixin_super_repeated.dart.weak.transformed.expect

@@ -8,10 +8,7 @@ abstract class M extends core::Object {
     : super core::Object::•()
     ;
 }
-abstract class N extends self::M {
-  synthetic constructor •() → self::N
-    : super self::M::•()
-    ;
+abstract class N extends self::M /*isMixinDeclaration*/  {
   set superM(dynamic value) → void {
     super.{self::M::m} = value;
   }

pkg/front_end/testcases/incremental/strongmode_mixins.yaml

@@ -17,5 +17,5 @@ sources:
     class A extends Object with B<Object>, C {}
   b.dart: |
     // @dart=2.9
-    abstract class C<T extends Object> extends Object with B<T> {}
+    mixin C<T extends Object> on B<T> {}
     abstract class B<ChildType extends Object> extends Object {}

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_1.dart

@@ -4,7 +4,7 @@
 // @dart=2.9
 class I<X> {}
 
-class M0<X, Y extends String> extends I<X> {}
+mixin M0<X, Y extends String> on I<X> {}
 
 class M1 implements I<int> {}
 

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_1.dart.textual_outline.expect

@@ -1,7 +1,7 @@
 // @dart = 2.9
 class I<X> {}
 
-class M0<X, Y extends String> extends I<X> {}
+mixin M0<X, Y extends String> on I<X> {}
 
 class M1 implements I<int> {}
 

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_1.dart.textual_outline_modelled.expect

@@ -3,8 +3,7 @@ class A extends M1 with M0 {}
 
 class I<X> {}
 
-class M0<X, Y extends String> extends I<X> {}
-
 class M1 implements I<int> {}
 
 main() {}
+mixin M0<X, Y extends String> on I<X> {}

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_1.dart.weak.expect

@@ -17,10 +17,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends core::String*> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::I::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends core::String*> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_1.dart.weak.modular.expect

@@ -17,10 +17,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends core::String*> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::I::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends core::String*> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_1.dart.weak.outline.expect

@@ -16,9 +16,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends core::String*> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends core::String*> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_1.dart.weak.transformed.expect

@@ -17,10 +17,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends core::String*> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::I::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends core::String*> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_2.dart

@@ -4,7 +4,7 @@
 // @dart=2.9
 class I<X> {}
 
-class M0<X, Y extends X> extends I<X> {}
+mixin M0<X, Y extends X> on I<X> {}
 
 class M1 implements I<int> {}
 

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_2.dart.textual_outline.expect

@@ -1,7 +1,7 @@
 // @dart = 2.9
 class I<X> {}
 
-class M0<X, Y extends X> extends I<X> {}
+mixin M0<X, Y extends X> on I<X> {}
 
 class M1 implements I<int> {}
 

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_2.dart.textual_outline_modelled.expect

@@ -3,8 +3,7 @@ class A extends M1 with M0 {}
 
 class I<X> {}
 
-class M0<X, Y extends X> extends I<X> {}
-
 class M1 implements I<int> {}
 
 main() {}
+mixin M0<X, Y extends X> on I<X> {}

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_2.dart.weak.expect

@@ -17,10 +17,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends self::M0::X* = dynamic> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::I::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends self::M0::X* = dynamic> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_2.dart.weak.modular.expect

@@ -17,10 +17,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends self::M0::X* = dynamic> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::I::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends self::M0::X* = dynamic> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_2.dart.weak.outline.expect

@@ -16,9 +16,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends self::M0::X* = dynamic> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends self::M0::X* = dynamic> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_2.dart.weak.transformed.expect

@@ -17,10 +17,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends self::M0::X* = dynamic> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::I::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends self::M0::X* = dynamic> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_3.dart

@@ -4,7 +4,7 @@
 // @dart=2.9
 class I<X> {}
 
-class M0<X, Y extends Comparable<Y>> extends I<X> {}
+mixin M0<X, Y extends Comparable<Y>> on I<X> {}
 
 class M1 implements I<int> {}
 

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_3.dart.textual_outline.expect

@@ -1,7 +1,7 @@
 // @dart = 2.9
 class I<X> {}
 
-class M0<X, Y extends Comparable<Y>> extends I<X> {}
+mixin M0<X, Y extends Comparable<Y>> on I<X> {}
 
 class M1 implements I<int> {}
 

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_3.dart.textual_outline_modelled.expect

@@ -3,8 +3,7 @@ class A extends M1 with M0 {}
 
 class I<X> {}
 
-class M0<X, Y extends Comparable<Y>> extends I<X> {}
-
 class M1 implements I<int> {}
 
 main() {}
+mixin M0<X, Y extends Comparable<Y>> on I<X> {}

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_3.dart.weak.expect

@@ -8,7 +8,7 @@ library;
 // class A extends M1 with M0 {}
 //                         ^
 // pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_3.dart:7:13: Context: This is the type variable whose bound isn't conformed to.
-// class M0<X, Y extends Comparable<Y>> extends I<X> {}
+// mixin M0<X, Y extends Comparable<Y>> on I<X> {}
 //             ^
 //
 import self as self;
@@ -29,10 +29,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends core::Comparable<self::M0::Y*>* = core::Comparable<dynamic>*> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::I::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends core::Comparable<self::M0::Y*>* = core::Comparable<dynamic>*> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_3.dart.weak.modular.expect

@@ -8,7 +8,7 @@ library;
 // class A extends M1 with M0 {}
 //                         ^
 // pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_3.dart:7:13: Context: This is the type variable whose bound isn't conformed to.
-// class M0<X, Y extends Comparable<Y>> extends I<X> {}
+// mixin M0<X, Y extends Comparable<Y>> on I<X> {}
 //             ^
 //
 import self as self;
@@ -29,10 +29,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends core::Comparable<self::M0::Y*>* = core::Comparable<dynamic>*> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::I::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends core::Comparable<self::M0::Y*>* = core::Comparable<dynamic>*> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_3.dart.weak.outline.expect

@@ -8,7 +8,7 @@ library;
 // class A extends M1 with M0 {}
 //                         ^
 // pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_3.dart:7:13: Context: This is the type variable whose bound isn't conformed to.
-// class M0<X, Y extends Comparable<Y>> extends I<X> {}
+// mixin M0<X, Y extends Comparable<Y>> on I<X> {}
 //             ^
 //
 import self as self;
@@ -28,9 +28,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends core::Comparable<self::M0::Y*>* = core::Comparable<dynamic>*> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends core::Comparable<self::M0::Y*>* = core::Comparable<dynamic>*> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_3.dart.weak.transformed.expect

@@ -8,7 +8,7 @@ library;
 // class A extends M1 with M0 {}
 //                         ^
 // pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_3.dart:7:13: Context: This is the type variable whose bound isn't conformed to.
-// class M0<X, Y extends Comparable<Y>> extends I<X> {}
+// mixin M0<X, Y extends Comparable<Y>> on I<X> {}
 //             ^
 //
 import self as self;
@@ -29,10 +29,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends core::Comparable<self::M0::Y*>* = core::Comparable<dynamic>*> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::I::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends core::Comparable<self::M0::Y*>* = core::Comparable<dynamic>*> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_4.dart

@@ -4,7 +4,7 @@
 // @dart=2.9
 class I<X> {}
 
-class M0<X, Y extends void Function({String name})> extends I<X> {}
+mixin M0<X, Y extends void Function({String name})> on I<X> {}
 
 class M1 implements I<int> {}
 

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_4.dart.textual_outline.expect

@@ -1,7 +1,7 @@
 // @dart = 2.9
 class I<X> {}
 
-class M0<X, Y extends void Function({String name})> extends I<X> {}
+mixin M0<X, Y extends void Function({String name})> on I<X> {}
 
 class M1 implements I<int> {}
 

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_4.dart.textual_outline_modelled.expect

@@ -3,8 +3,7 @@ class A extends M1 with M0 {}
 
 class I<X> {}
 
-class M0<X, Y extends void Function({String name})> extends I<X> {}
-
 class M1 implements I<int> {}
 
 main() {}
+mixin M0<X, Y extends void Function({String name})> on I<X> {}

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_4.dart.weak.expect

@@ -17,10 +17,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends ({name: core::String*}) →* void> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::I::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends ({name: core::String*}) →* void> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_4.dart.weak.modular.expect

@@ -17,10 +17,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends ({name: core::String*}) →* void> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::I::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends ({name: core::String*}) →* void> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_4.dart.weak.outline.expect

@@ -16,9 +16,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends ({name: core::String*}) →* void> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends ({name: core::String*}) →* void> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_instantiate_to_bounds_4.dart.weak.transformed.expect

@@ -17,10 +17,7 @@ class I<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends ({name: core::String*}) →* void> extends self::I<self::M0::X*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::I::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends ({name: core::String*}) →* void> extends self::I<self::M0::X*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_multiple_constraints.dart

@@ -6,7 +6,7 @@ class I<X> {}
 
 class J<X> {}
 
-class M0<X, Y> extends I<X> with J<Y> {}
+mixin M0<X, Y> on I<X>, J<Y> {}
 
 class M1 implements I<int> {}
 

pkg/front_end/testcases/inference/mixin_inference_multiple_constraints.dart.textual_outline.expect

@@ -3,7 +3,7 @@ class I<X> {}
 
 class J<X> {}
 
-class M0<X, Y> extends I<X> with J<Y> {}
+mixin M0<X, Y> on I<X>, J<Y> {}
 
 class M1 implements I<int> {}
 

pkg/front_end/testcases/inference/mixin_inference_multiple_constraints.dart.textual_outline_modelled.expect

@@ -5,10 +5,9 @@ class I<X> {}
 
 class J<X> {}
 
-class M0<X, Y> extends I<X> with J<Y> {}
-
 class M1 implements I<int> {}
 
 class M2 extends M1 implements J<double> {}
 
 main() {}
+mixin M0<X, Y> on I<X>, J<Y> {}

pkg/front_end/testcases/inference/mixin_inference_multiple_constraints.dart.weak.expect

@@ -32,15 +32,22 @@ class J<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-abstract class _M0&I&J<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> = self::I<self::_M0&I&J::X*> with self::J<self::_M0&I&J::Y*> /*isAnonymousMixin*/  {
+abstract class _M0&I&J<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> extends core::Object implements self::I<self::_M0&I&J::X*>, self::J<self::_M0&I&J::Y*> /*isAnonymousMixin*/  {
   synthetic constructor •() → self::_M0&I&J<self::_M0&I&J::X*, self::_M0&I&J::Y*>*
-    : super self::I::•()
+    : super core::Object::•()
     ;
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> extends self::_M0&I&J<self::M0::X*, self::M0::Y*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::_M0&I&J::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> extends self::_M0&I&J<self::M0::X*, self::M0::Y*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_multiple_constraints.dart.weak.modular.expect

@@ -32,15 +32,22 @@ class J<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-abstract class _M0&I&J<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> = self::I<self::_M0&I&J::X*> with self::J<self::_M0&I&J::Y*> /*isAnonymousMixin*/  {
+abstract class _M0&I&J<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> extends core::Object implements self::I<self::_M0&I&J::X*>, self::J<self::_M0&I&J::Y*> /*isAnonymousMixin*/  {
   synthetic constructor •() → self::_M0&I&J<self::_M0&I&J::X*, self::_M0&I&J::Y*>*
-    : super self::I::•()
+    : super core::Object::•()
     ;
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> extends self::_M0&I&J<self::M0::X*, self::M0::Y*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::_M0&I&J::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> extends self::_M0&I&J<self::M0::X*, self::M0::Y*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_multiple_constraints.dart.weak.outline.expect

@@ -30,14 +30,21 @@ class J<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-abstract class _M0&I&J<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> = self::I<self::_M0&I&J::X*> with self::J<self::_M0&I&J::Y*> /*isAnonymousMixin*/  {
+abstract class _M0&I&J<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> extends core::Object implements self::I<self::_M0&I&J::X*>, self::J<self::_M0&I&J::Y*> /*isAnonymousMixin*/  {
   synthetic constructor •() → self::_M0&I&J<self::_M0&I&J::X*, self::_M0&I&J::Y*>*
-    : super self::I::•()
     ;
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> extends self::_M0&I&J<self::M0::X*, self::M0::Y*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> extends self::_M0&I&J<self::M0::X*, self::M0::Y*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_multiple_constraints.dart.weak.transformed.expect

@@ -32,15 +32,22 @@ class J<X extends core::Object* = dynamic> extends core::Object {
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-abstract class _M0&I&J<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> extends self::I<self::_M0&I&J::X*> implements self::J<self::_M0&I&J::Y*> /*isAnonymousMixin,isEliminatedMixin*/  {
+abstract class _M0&I&J<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> extends core::Object implements self::I<self::_M0&I&J::X*>, self::J<self::_M0&I&J::Y*> /*isAnonymousMixin*/  {
   synthetic constructor •() → self::_M0&I&J<self::_M0&I&J::X*, self::_M0&I&J::Y*>*
-    : super self::I::•()
+    : super core::Object::•()
     ;
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M0<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> extends self::_M0&I&J<self::M0::X*, self::M0::Y*> {
-  synthetic constructor •() → self::M0<self::M0::X*, self::M0::Y*>*
-    : super self::_M0&I&J::•()
-    ;
+abstract class M0<X extends core::Object* = dynamic, Y extends core::Object* = dynamic> extends self::_M0&I&J<self::M0::X*, self::M0::Y*> /*isMixinDeclaration*/  {
 }
 class M1 extends core::Object implements self::I<core::int*> {
   synthetic constructor •() → self::M1*

pkg/front_end/testcases/inference/mixin_inference_non_trivial_constraints.dart.weak.expect

@@ -1,4 +1,11 @@
 library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/inference/mixin_inference_non_trivial_constraints.dart:14:30: Error: The class 'M0' can't be used as a mixin because it extends a class other than 'Object'.
+// class A extends M2<int> with M0 {}
+//                              ^
+//
 import self as self;
 import "dart:core" as core;
 

pkg/front_end/testcases/inference/mixin_inference_non_trivial_constraints.dart.weak.modular.expect

@@ -1,4 +1,11 @@
 library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/inference/mixin_inference_non_trivial_constraints.dart:14:30: Error: The class 'M0' can't be used as a mixin because it extends a class other than 'Object'.
+// class A extends M2<int> with M0 {}
+//                              ^
+//
 import self as self;
 import "dart:core" as core;
 

pkg/front_end/testcases/inference/mixin_inference_non_trivial_constraints.dart.weak.outline.expect

@@ -1,4 +1,11 @@
 library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/inference/mixin_inference_non_trivial_constraints.dart:14:30: Error: The class 'M0' can't be used as a mixin because it extends a class other than 'Object'.
+// class A extends M2<int> with M0 {}
+//                              ^
+//
 import self as self;
 import "dart:core" as core;
 

pkg/front_end/testcases/inference/mixin_inference_non_trivial_constraints.dart.weak.transformed.expect

@@ -1,4 +1,11 @@
 library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/inference/mixin_inference_non_trivial_constraints.dart:14:30: Error: The class 'M0' can't be used as a mixin because it extends a class other than 'Object'.
+// class A extends M2<int> with M0 {}
+//                              ^
+//
 import self as self;
 import "dart:core" as core;
 

pkg/front_end/testcases/inference/mixin_inference_outwards_1.dart

@@ -6,7 +6,7 @@ class I<X> {}
 
 class M0<T> extends I<T> {}
 
-class M1<T> extends I<T> {}
+mixin M1<T> on I<T> {}
 
 // M1 is inferred as M1<int>
 class A extends M0<int> with M1 {}

pkg/front_end/testcases/inference/mixin_inference_outwards_1.dart.textual_outline.expect

@@ -3,7 +3,7 @@ class I<X> {}
 
 class M0<T> extends I<T> {}
 
-class M1<T> extends I<T> {}
+mixin M1<T> on I<T> {}
 
 class A extends M0<int> with M1 {}
 

pkg/front_end/testcases/inference/mixin_inference_outwards_1.dart.textual_outline_modelled.expect

@@ -5,6 +5,5 @@ class I<X> {}
 
 class M0<T> extends I<T> {}
 
-class M1<T> extends I<T> {}
-
 main() {}
+mixin M1<T> on I<T> {}

pkg/front_end/testcases/inference/mixin_inference_outwards_1.dart.weak.expect

@@ -22,10 +22,7 @@ class M0<T extends core::Object* = dynamic> extends self::I<self::M0::T*> {
     : super self::I::•()
     ;
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    : super self::I::•()
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&M0&M1 = self::M0<core::int*> with self::M1<core::int*> /*isAnonymousMixin*/  {
   synthetic constructor •() → self::_A&M0&M1*

pkg/front_end/testcases/inference/mixin_inference_outwards_1.dart.weak.modular.expect

@@ -22,10 +22,7 @@ class M0<T extends core::Object* = dynamic> extends self::I<self::M0::T*> {
     : super self::I::•()
     ;
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    : super self::I::•()
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&M0&M1 = self::M0<core::int*> with self::M1<core::int*> /*isAnonymousMixin*/  {
   synthetic constructor •() → self::_A&M0&M1*

pkg/front_end/testcases/inference/mixin_inference_outwards_1.dart.weak.outline.expect

@@ -20,9 +20,7 @@ class M0<T extends core::Object* = dynamic> extends self::I<self::M0::T*> {
   synthetic constructor •() → self::M0<self::M0::T*>*
     ;
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&M0&M1 = self::M0<core::int*> with self::M1<core::int*> /*isAnonymousMixin*/  {
   synthetic constructor •() → self::_A&M0&M1*

pkg/front_end/testcases/inference/mixin_inference_outwards_1.dart.weak.transformed.expect

@@ -22,10 +22,7 @@ class M0<T extends core::Object* = dynamic> extends self::I<self::M0::T*> {
     : super self::I::•()
     ;
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    : super self::I::•()
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&M0&M1 extends self::M0<core::int*> implements self::M1<core::int*> /*isAnonymousMixin,isEliminatedMixin*/  {
   synthetic constructor •() → self::_A&M0&M1*

pkg/front_end/testcases/inference/mixin_inference_outwards_2.dart

@@ -6,9 +6,9 @@ class I<X> {}
 
 class M0<T> extends I<T> {}
 
-class M1<T> extends I<T> {}
+mixin M1<T> on I<T> {}
 
-class M2<T> extends I<T> {}
+mixin M2<T> on I<T> {}
 
 // M1 is inferred as M1<int>
 // M2 is inferred as M1<int>

pkg/front_end/testcases/inference/mixin_inference_outwards_2.dart.textual_outline.expect

@@ -3,9 +3,8 @@ class I<X> {}
 
 class M0<T> extends I<T> {}
 
-class M1<T> extends I<T> {}
-
-class M2<T> extends I<T> {}
+mixin M1<T> on I<T> {}
+mixin M2<T> on I<T> {}
 
 class A extends M0<int> with M1, M2 {}
 

pkg/front_end/testcases/inference/mixin_inference_outwards_2.dart.textual_outline_modelled.expect

@@ -5,8 +5,6 @@ class I<X> {}
 
 class M0<T> extends I<T> {}
 
-class M1<T> extends I<T> {}
-
-class M2<T> extends I<T> {}
-
 main() {}
+mixin M1<T> on I<T> {}
+mixin M2<T> on I<T> {}

pkg/front_end/testcases/inference/mixin_inference_outwards_2.dart.weak.expect

@@ -22,15 +22,9 @@ class M0<T extends core::Object* = dynamic> extends self::I<self::M0::T*> {
     : super self::I::•()
     ;
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    : super self::I::•()
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
-class M2<T extends core::Object* = dynamic> extends self::I<self::M2::T*> {
-  synthetic constructor •() → self::M2<self::M2::T*>*
-    : super self::I::•()
-    ;
+abstract class M2<T extends core::Object* = dynamic> extends self::I<self::M2::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&M0&M1 = self::M0<core::int*> with self::M1<core::int*> /*isAnonymousMixin*/  {
   synthetic constructor •() → self::_A&M0&M1*

pkg/front_end/testcases/inference/mixin_inference_outwards_2.dart.weak.modular.expect

@@ -22,15 +22,9 @@ class M0<T extends core::Object* = dynamic> extends self::I<self::M0::T*> {
     : super self::I::•()
     ;
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    : super self::I::•()
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
-class M2<T extends core::Object* = dynamic> extends self::I<self::M2::T*> {
-  synthetic constructor •() → self::M2<self::M2::T*>*
-    : super self::I::•()
-    ;
+abstract class M2<T extends core::Object* = dynamic> extends self::I<self::M2::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&M0&M1 = self::M0<core::int*> with self::M1<core::int*> /*isAnonymousMixin*/  {
   synthetic constructor •() → self::_A&M0&M1*

pkg/front_end/testcases/inference/mixin_inference_outwards_2.dart.weak.outline.expect

@@ -20,13 +20,9 @@ class M0<T extends core::Object* = dynamic> extends self::I<self::M0::T*> {
   synthetic constructor •() → self::M0<self::M0::T*>*
     ;
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
-class M2<T extends core::Object* = dynamic> extends self::I<self::M2::T*> {
-  synthetic constructor •() → self::M2<self::M2::T*>*
-    ;
+abstract class M2<T extends core::Object* = dynamic> extends self::I<self::M2::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&M0&M1 = self::M0<core::int*> with self::M1<core::int*> /*isAnonymousMixin*/  {
   synthetic constructor •() → self::_A&M0&M1*

pkg/front_end/testcases/inference/mixin_inference_outwards_2.dart.weak.transformed.expect

@@ -22,15 +22,9 @@ class M0<T extends core::Object* = dynamic> extends self::I<self::M0::T*> {
     : super self::I::•()
     ;
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    : super self::I::•()
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
-class M2<T extends core::Object* = dynamic> extends self::I<self::M2::T*> {
-  synthetic constructor •() → self::M2<self::M2::T*>*
-    : super self::I::•()
-    ;
+abstract class M2<T extends core::Object* = dynamic> extends self::I<self::M2::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&M0&M1 extends self::M0<core::int*> implements self::M1<core::int*> /*isAnonymousMixin,isEliminatedMixin*/  {
   synthetic constructor •() → self::_A&M0&M1*

pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart

@@ -6,7 +6,7 @@ class I<X> {}
 
 class M0<T> extends Object implements I<T> {}
 
-class M1<T> extends I<T> {}
+mixin M1<T> on I<T> {}
 
 // M0 is inferred as M0<dynamic>
 // Error since class hierarchy is inconsistent

pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart.textual_outline.expect

@@ -3,7 +3,7 @@ class I<X> {}
 
 class M0<T> extends Object implements I<T> {}
 
-class M1<T> extends I<T> {}
+mixin M1<T> on I<T> {}
 
 class A extends Object with M0, M1<int> {}
 

pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart.textual_outline_modelled.expect

@@ -5,6 +5,5 @@ class I<X> {}
 
 class M0<T> extends Object implements I<T> {}
 
-class M1<T> extends I<T> {}
-
 main() {}
+mixin M1<T> on I<T> {}

pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart.weak.expect

@@ -7,6 +7,16 @@
 //       ^
 //
 library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart:13:7: Error: '_A&Object&M0' doesn't implement 'I<int>' so it can't be used with 'M1<int>'.
+//  - 'Object with M0' is from 'pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart'.
+//  - 'I' is from 'pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart'.
+//  - 'M1' is from 'pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart'.
+// class A extends Object with M0, M1<int> {}
+//       ^
+//
 import self as self;
 import "dart:core" as core;
 
@@ -40,10 +50,7 @@ class M0<T extends core::Object* = dynamic> extends core::Object implements self
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    : super self::I::•()
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&Object&M0 = core::Object with self::M0<dynamic> /*isAnonymousMixin,hasConstConstructor*/  {
   const synthetic constructor •() → self::_A&Object&M0*

pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart.weak.modular.expect

@@ -7,6 +7,16 @@
 //       ^
 //
 library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart:13:7: Error: '_A&Object&M0' doesn't implement 'I<int>' so it can't be used with 'M1<int>'.
+//  - 'Object with M0' is from 'pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart'.
+//  - 'I' is from 'pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart'.
+//  - 'M1' is from 'pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart'.
+// class A extends Object with M0, M1<int> {}
+//       ^
+//
 import self as self;
 import "dart:core" as core;
 
@@ -40,10 +50,7 @@ class M0<T extends core::Object* = dynamic> extends core::Object implements self
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    : super self::I::•()
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&Object&M0 = core::Object with self::M0<dynamic> /*isAnonymousMixin,hasConstConstructor*/  {
   const synthetic constructor •() → self::_A&Object&M0*

pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart.weak.outline.expect

@@ -7,6 +7,16 @@
 //       ^
 //
 library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart:13:7: Error: '_A&Object&M0' doesn't implement 'I<int>' so it can't be used with 'M1<int>'.
+//  - 'Object with M0' is from 'pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart'.
+//  - 'I' is from 'pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart'.
+//  - 'M1' is from 'pkg/front_end/testcases/inference/mixin_inference_outwards_3.dart'.
+// class A extends Object with M0, M1<int> {}
+//       ^
+//
 import self as self;
 import "dart:core" as core;
 
@@ -38,9 +48,7 @@ class M0<T extends core::Object* = dynamic> extends core::Object implements self
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&Object&M0 = core::Object with self::M0<dynamic> /*isAnonymousMixin,hasConstConstructor*/  {
   const synthetic constructor •() → self::_A&Object&M0*

pkg/front_end/testcases/inference/mixin_inference_outwards_4.dart

@@ -6,7 +6,7 @@ class I<X> {}
 
 class M0<T> extends Object implements I<T> {}
 
-class M1<T> extends I<T> {}
+mixin M1<T> on I<T> {}
 
 // M0 is inferred as M0<dynamic> (unconstrained)
 // M1 is inferred as M1<dynamic> (constrained by inferred argument to M0)

pkg/front_end/testcases/inference/mixin_inference_outwards_4.dart.textual_outline.expect

@@ -3,7 +3,7 @@ class I<X> {}
 
 class M0<T> extends Object implements I<T> {}
 
-class M1<T> extends I<T> {}
+mixin M1<T> on I<T> {}
 
 class A extends Object with M0, M1 implements I<int> {}
 

pkg/front_end/testcases/inference/mixin_inference_outwards_4.dart.textual_outline_modelled.expect

@@ -5,6 +5,5 @@ class I<X> {}
 
 class M0<T> extends Object implements I<T> {}
 
-class M1<T> extends I<T> {}
-
 main() {}
+mixin M1<T> on I<T> {}

pkg/front_end/testcases/inference/mixin_inference_outwards_4.dart.weak.expect

@@ -40,10 +40,7 @@ class M0<T extends core::Object* = dynamic> extends core::Object implements self
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    : super self::I::•()
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&Object&M0 = core::Object with self::M0<dynamic> /*isAnonymousMixin,hasConstConstructor*/  {
   const synthetic constructor •() → self::_A&Object&M0*

pkg/front_end/testcases/inference/mixin_inference_outwards_4.dart.weak.modular.expect

@@ -40,10 +40,7 @@ class M0<T extends core::Object* = dynamic> extends core::Object implements self
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    : super self::I::•()
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&Object&M0 = core::Object with self::M0<dynamic> /*isAnonymousMixin,hasConstConstructor*/  {
   const synthetic constructor •() → self::_A&Object&M0*

pkg/front_end/testcases/inference/mixin_inference_outwards_4.dart.weak.outline.expect

@@ -38,9 +38,7 @@ class M0<T extends core::Object* = dynamic> extends core::Object implements self
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
   abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
 }
-class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> {
-  synthetic constructor •() → self::M1<self::M1::T*>*
-    ;
+abstract class M1<T extends core::Object* = dynamic> extends self::I<self::M1::T*> /*isMixinDeclaration*/  {
 }
 abstract class _A&Object&M0 = core::Object with self::M0<dynamic> /*isAnonymousMixin,hasConstConstructor*/  {
   const synthetic constructor •() → self::_A&Object&M0*

pkg/vm/test/incremental_compiler_test.dart

@@ -302,7 +302,7 @@ main() {
       main.writeAsStringSync("""
       import 'lib.dart';
       main() => print(foo());
-      class C1 extends Object with C2, C3 {
+      class C1 extends Object with C3, C2 {
         c1method() {
           print("c1");
         }
@@ -318,7 +318,7 @@ main() {
       import 'main.dart';
       foo() => 'foo';
       main() => print('bar');
-      class C2 extends Object with C3 {
+      mixin C2 on C3 {
         c2method() {
           print("c2");
         }

pkg/vm/testcases/transformations/to_string_transformer/lib/main.dart

@@ -30,9 +30,9 @@ class Keep {
 @keepToStringInSubtypes
 class Base1 {}
 
-class Base2 extends Base1 {}
+mixin Base2 on Base1 {}
 
-class Base3 extends Object with Base2 {}
+class Base3 extends Object with Base1, Base2 {}
 
 class KeepInherited implements Base3 {
   @override