[tests] Update language tests with base/final subtyping loophole fixed and a few enum tests.

Change-Id: Id30bbdc4b6eedf0751f725024acbed812a07a4fc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/280860
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>

tests/language/class_modifiers/base/base_class_abstract_construct_error_test.dart

@@ -13,11 +13,11 @@ abstract base class AlsoNotConstructable = Object with M;
 
 main() {
   var error = NotConstructable();
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
+//            ^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INSTANTIATE_ABSTRACT_CLASS
+// [cfe] The class 'NotConstructable' is abstract and can't be instantiated.
   var error2 = AlsoNotConstructable();
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
+//             ^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INSTANTIATE_ABSTRACT_CLASS
+// [cfe] The class 'AlsoNotConstructable' is abstract and can't be instantiated.
 }

tests/language/class_modifiers/base/base_class_base_subtype_test.dart

@@ -13,7 +13,6 @@ base class BaseClass {
 }
 base class A extends BaseClass {}
 base class B implements BaseClass {
-  @override
   int foo = 1;
 }
 
@@ -24,14 +23,14 @@ base class AMixin with BaseMixin {}
 base class BMixin = Object with BaseMixin;
 
 // Used for trivial runtime tests of the base subtypes.
-class AConcrete extends A {}
-class BConcrete extends B {}
-class AMixinConcrete extends AMixin {}
-class BMixinConcrete extends BMixin {}
+base class AConcrete extends A {}
+base class BConcrete extends B {}
+base class AMixinConcrete extends AMixin {}
+base class BMixinConcrete extends BMixin {}
 
 main() {
   Expect.equals(0, AConcrete().foo);
   Expect.equals(1, BConcrete().foo);
   Expect.equals(0, AMixinConcrete().foo);
   Expect.equals(0, BMixinConcrete().foo);
-}
+}

tests/language/class_modifiers/base/base_class_extend_lib.dart

@@ -10,6 +10,6 @@ base class BaseClass {
   int foo = 0;
 }
 
-abstract class A extends BaseClass {}
+abstract base class A extends BaseClass {}
 
-class B extends BaseClass {}
+base class B extends BaseClass {}

tests/language/class_modifiers/base/base_class_extend_not_base_final_error_test.dart

@@ -4,8 +4,8 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
-// Error when extending a base class where the subclass is not also a base class
-// or final.
+// Error when extending a base class where the subclass is not a base, final or
+// sealed class.
 
 import 'base_class_extend_lib.dart';
 

tests/language/class_modifiers/base/base_class_extend_test.dart

@@ -10,7 +10,7 @@
 import 'package:expect/expect.dart';
 import 'base_class_extend_lib.dart';
 
-class AImpl extends A {}
+base class AImpl extends A {}
 
 main() {
   Expect.equals(0, AImpl().foo);

tests/language/class_modifiers/base/base_class_implement_error_test.dart

@@ -8,15 +8,19 @@
 
 import 'base_class_implement_lib.dart';
 
-abstract class AOutside implements BaseClass {}
-//                                 ^^^^^^^^^
+abstract base class AOutside implements BaseClass {}
+//                                      ^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
 
-class BOutside implements BaseClass {
-//                        ^^^^^^^^^
+base class BOutside implements BaseClass {
+//                             ^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
-  @override
   int foo = 1;
 }
+
+enum EnumOutside implements ClassForEnum { x }
+//                          ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'ClassForEnum' can't be implemented outside of its library because it's a base class.

tests/language/class_modifiers/base/base_class_implement_lib.dart

@@ -10,9 +10,16 @@ base class BaseClass {
   int foo = 0;
 }
 
-abstract class A implements BaseClass {}
+base class ClassForEnum {}
 
-class B implements BaseClass {
-  @override
+abstract base class A implements BaseClass {}
+
+base class AImpl implements A {
   int foo = 1;
 }
+
+base class B implements BaseClass {
+  int foo = 1;
+}
+
+enum EnumInside implements ClassForEnum { x }

tests/language/class_modifiers/base/base_class_implement_test.dart

@@ -10,12 +10,8 @@
 import 'package:expect/expect.dart';
 import 'base_class_implement_lib.dart';
 
-class AImpl implements A {
-  @override
-  int foo = 1;
-}
-
 main() {
   Expect.equals(1, AImpl().foo);
   Expect.equals(1, B().foo);
+  Expect.equals(0, EnumInside.x.index);
 }

tests/language/class_modifiers/base/base_class_inside_not_base_final_sealed_error_test.dart

@@ -0,0 +1,36 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers
+
+// Error when subtyping a base class where the subtype is not base, final or
+// sealed.
+
+base class BaseClass {}
+base mixin BaseMixin {}
+
+class Extends extends BaseClass {}
+// ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+class Implements implements BaseClass {}
+// ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+mixin MixinImplements implements BaseMixin {}
+// ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+class With with BaseMixin {}
+// ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+mixin On on BaseClass {}
+// ^
+// [analyzer] unspecified
+// [cfe] unspecified

tests/language/class_modifiers/base/base_class_mixin_on_lib.dart

@@ -4,14 +4,16 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by base_class_mixin_on_test.dart.
+
 base class BaseClass {}
 
-abstract class A extends BaseClass {}
+abstract base class A extends BaseClass {}
 
-class B extends BaseClass {}
+base class B extends BaseClass {}
 
 base mixin BaseMixin {}
 
-class C extends BaseClass with BaseMixin {}
+base class C extends BaseClass with BaseMixin {}
 
-class D with BaseMixin {}
+base class D with BaseMixin {}

tests/language/class_modifiers/base/base_class_mixin_on_test.dart

@@ -10,22 +10,22 @@
 import 'package:expect/expect.dart';
 import 'base_class_mixin_on_lib.dart';
 
-mixin MA on BaseClass {}
-mixin MB on BaseClass {}
+base mixin MA on BaseClass {}
+base mixin MB on BaseClass {}
 
-class ConcreteA extends A with MA, MB {
+base class ConcreteA extends A with MA, MB {
   int foo = 0;
 }
 
-mixin MC on BaseClass, BaseMixin {}
+base mixin MC on BaseClass, BaseMixin {}
 
-class ConcreteC extends C with MC {
+base class ConcreteC extends C with MC {
   int foo = 0;
 }
 
-mixin MCSingular on BaseMixin {}
+base mixin MCSingular on BaseMixin {}
 
-class ConcreteD extends D with MCSingular {
+base class ConcreteD extends D with MCSingular {
   int foo = 0;
 }
 

tests/language/class_modifiers/base/base_class_part_lib.dart

@@ -4,11 +4,12 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other declarations used by base_class_part_test.dart.
+
 part of 'base_class_part_test.dart';
 
-class A extends BaseClass {}
+base class A extends BaseClass {}
 
-class B implements BaseClass {
-  @override
+base class B implements BaseClass {
   int foo = 1;
 }

tests/language/class_modifiers/base/base_class_subtype_base_final_or_sealed_test.dart

@@ -0,0 +1,74 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers,sealed-class
+
+// Allow base mixins/classes to be subtyped by base, final, or sealed
+// classes/mixins and produce no error.
+
+import 'package:expect/expect.dart';
+
+base class BaseClass {
+  int foo = 0;
+}
+base mixin BaseMixin {
+  int foo = 0;
+}
+
+base class BaseExtends extends BaseClass {}
+final class FinalExtends extends BaseClass {}
+sealed class SealedExtends extends BaseClass {}
+base class SealedExtendsImpl extends SealedExtends {}
+
+base class BaseImplements implements BaseClass {
+  int foo = 0;
+}
+final class FinalImplements implements BaseClass {
+  int foo = 0;
+}
+sealed class SealedImplements implements BaseClass {
+  int foo = 0;
+}
+base class SealedImplementsImpl extends SealedImplements {}
+
+base mixin BaseMixinImplements implements BaseMixin {}
+final mixin FinalMixinImplements implements BaseMixin {}
+sealed mixin SealedMixinImplements implements BaseMixin {}
+
+base class ImplementsImpl implements BaseMixinImplements, FinalMixinImplements, SealedMixinImplements {
+  int foo = 0;
+}
+
+base class BaseWith with BaseMixin {}
+final class FinalWith with BaseMixin {}
+sealed class SealedWith with BaseMixin {}
+base class SealedWithImpl extends SealedWith {}
+
+base mixin BaseOn on BaseClass {}
+final mixin FinalOn on BaseClass {}
+sealed mixin SealedOn on BaseClass {}
+
+base class OnImpl implements BaseOn, FinalOn, SealedOn {
+  int foo = 0;
+}
+
+base mixin MixinForEnum {}
+enum EnumWith with MixinForEnum { x }
+enum EnumImplements implements MixinForEnum { x }
+
+main() {
+  Expect.equals(0, BaseExtends().foo);
+  Expect.equals(0, FinalExtends().foo);
+  Expect.equals(0, SealedExtendsImpl().foo);
+  Expect.equals(0, BaseImplements().foo);
+  Expect.equals(0, FinalImplements().foo);
+  Expect.equals(0, SealedImplementsImpl().foo);
+  Expect.equals(0, ImplementsImpl().foo);
+  Expect.equals(0, BaseWith().foo);
+  Expect.equals(0, FinalWith().foo);
+  Expect.equals(0, SealedWithImpl().foo);
+  Expect.equals(0, OnImpl().foo);
+  Expect.equals(0, EnumWith.x.index);
+  Expect.equals(0, EnumImplements.x.index);
+}

tests/language/class_modifiers/base/base_class_syntax_disabled_error_test.dart

@@ -1,22 +0,0 @@
-// 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.
-
-// Make sure errors are emitted when trying to use base classes without
-// the `class-modifiers` experiment enabled.
-
-base class BaseClass {}
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
-
-mixin M {}
-base class BaseClassTypeAlias = Object with M;
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
-
-base mixin BaseMixin {}
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified

tests/language/class_modifiers/base/base_class_syntax_error_test.dart

@@ -9,37 +9,89 @@
 
 abstract class BaseMembers {
   base int foo;
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
+//^^^^
+// [analyzer] COMPILE_TIME_ERROR.IMPLICIT_THIS_REFERENCE_IN_INITIALIZER
+// [analyzer] COMPILE_TIME_ERROR.NOT_A_TYPE
+// [cfe] 'base' isn't a type.
+//     ^^^
+// [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
+// [cfe] Expected ';' after this.
+//         ^^^
+// [analyzer] SYNTACTIC_ERROR.MISSING_CONST_FINAL_VAR_OR_TYPE
+// [cfe] Variables must be declared using the keywords 'const', 'final', 'var' or a type name.
 
   int bar(base int x);
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
+//^^^
+// [analyzer] COMPILE_TIME_ERROR.NOT_A_TYPE
+// [cfe] 'int' isn't a type.
+//        ^^^^
+// [analyzer] COMPILE_TIME_ERROR.NOT_A_TYPE
+// [cfe] 'base' isn't a type.
+//                 ^
+// [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
+// [cfe] Expected ')' before this.
 
   base void bar2();
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
+//^^^^
+// [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
+// [analyzer] SYNTACTIC_ERROR.MISSING_CONST_FINAL_VAR_OR_TYPE
+// [cfe] Expected ';' after this.
+// [cfe] Variables must be declared using the keywords 'const', 'final', 'var' or a type name.
 }
 
 base base class BaseDuplicateClass {}
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
+// [error column 1, length 4]
+// [analyzer] COMPILE_TIME_ERROR.NOT_A_TYPE
+// [cfe] 'base' isn't a type.
+// [cfe] Can't use 'base' because it is declared more than once.
+//   ^^^^
+// [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
+// [cfe] Expected ';' after this.
 
 base abstract class BaseAbstractClass {}
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
+// [error column 1, length 4]
+// [analyzer] COMPILE_TIME_ERROR.NOT_A_TYPE
+// [cfe] 'base' isn't a type.
+// [cfe] Can't use 'base' because it is declared more than once.
+//   ^^^^^^^^
+// [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
+// [cfe] Expected ';' after this.
 
 class BaseVariable {
   int foo() {
     base var x = 2;
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
+//  ^^^^
+// [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
+// [cfe] Can't use 'base' because it is declared more than once.
+// [cfe] Expected ';' after this.
     return x;
   }
-}
+}
+
+base extension StringExtension on String {}
+// [error column 1, length 4]
+// [analyzer] COMPILE_TIME_ERROR.NOT_A_TYPE
+// [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
+// [cfe] 'base' isn't a type.
+// [cfe] Expected ';' after this.
+//   ^^^^^^^^^
+// [analyzer] SYNTACTIC_ERROR.MISSING_IDENTIFIER
+// [cfe] Expected an identifier, but got 'extension'.
+
+base enum Enum { x }
+// [error column 1, length 4]
+// [analyzer] COMPILE_TIME_ERROR.DUPLICATE_DEFINITION
+// [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
+// [analyzer] SYNTACTIC_ERROR.MISSING_CONST_FINAL_VAR_OR_TYPE
+// [cfe] 'base' is already declared in this scope.
+// [cfe] Expected ';' after this.
+// [cfe] Variables must be declared using the keywords 'const', 'final', 'var' or a type name.
+
+base typedef EnumTypedef = Enum;
+// [error column 1, length 4]
+// [analyzer] COMPILE_TIME_ERROR.DUPLICATE_DEFINITION
+// [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
+// [analyzer] SYNTACTIC_ERROR.MISSING_CONST_FINAL_VAR_OR_TYPE
+// [cfe] 'base' is already declared in this scope.
+// [cfe] Expected ';' after this.
+// [cfe] Variables must be declared using the keywords 'const', 'final', 'var' or a type name.

tests/language/class_modifiers/base/base_class_typedef_implement_error_test.dart

@@ -8,21 +8,19 @@
 
 import 'base_class_typedef_lib.dart';
 
-class BTypeDef implements BaseClassTypeDef {
-//                        ^^^^^^^^^^^^^^^^
+base class BTypeDef implements BaseClassTypeDef {
+//                             ^^^^^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
-  @override
   int foo = 1;
 }
 
 // Testing another layer of typedefs outside of the library.
 typedef BaseClassTypeDef2 = BaseClassTypeDef;
 
-class BTypeDef2 implements BaseClassTypeDef2 {
-//                         ^^^^^^^^^^^^^^^^^
+base class BTypeDef2 implements BaseClassTypeDef2 {
+//                              ^^^^^^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
-  @override
   int foo = 1;
 }

tests/language/class_modifiers/base/base_class_typedef_lib.dart

@@ -4,15 +4,17 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// base_class_typedef_implement_error_test.dart.
+
 base class BaseClass {
   int foo = 0;
 }
 
 typedef BaseClassTypeDef = BaseClass;
 
-class A extends BaseClassTypeDef {}
+base class A extends BaseClassTypeDef {}
 
-class B implements BaseClassTypeDef {
-  @override
+base class B implements BaseClassTypeDef {
   int foo = 1;
 }

tests/language/class_modifiers/base/base_class_typedef_outside_of_library_lib.dart

@@ -4,14 +4,17 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by 
+// base_class_typedef_outside_of_library_test.dart
+
 import 'base_class_typedef_outside_of_library_lib2.dart';
 
 base class BaseClass {
   int foo = 0;
 }
 
-class A extends ATypeDef {}
+base class A extends ATypeDef {}
 
-class B implements ATypeDef {
+base class B implements ATypeDef {
   int foo = 1;
 }

tests/language/class_modifiers/base/base_class_typedef_outside_of_library_lib2.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// base_class_typedef_outside_of_library_test.dart
+
 import "base_class_typedef_outside_of_library_lib.dart";
 
 typedef ATypeDef = BaseClass;

tests/language/class_modifiers/base/base_class_typedef_used_outside_error_test.dart

@@ -12,8 +12,8 @@ import 'base_class_typedef_used_outside_lib.dart';
 
 typedef ATypeDef = BaseClass;
 
-class B implements ATypeDef {
-//                 ^^^^^^^^
+base class B implements ATypeDef {
+//                      ^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'BaseClass' can't be implemented outside of its library because it's a base class.
   int foo = 1;

tests/language/class_modifiers/base/base_class_typedef_used_outside_lib.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by 
+// base_class_typedef_used_outside_error_test.dart.
+
 base class BaseClass {
   int foo = 0;
 }

tests/language/class_modifiers/base/base_mixin_implement_error_test.dart

@@ -8,15 +8,19 @@
 
 import 'base_mixin_implement_lib.dart';
 
-abstract class AOutside implements BaseMixin {}
-//                                 ^^^^^^^^^
+abstract base class AOutside implements BaseMixin {}
+//                                      ^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
 
-class BOutside implements BaseMixin {
-//                        ^^^^^^^^^
+base class BOutside implements BaseMixin {
+//                             ^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The mixin 'BaseMixin' can't be implemented outside of its library because it's a base mixin.
-  @override
   int foo = 1;
 }
+
+enum EnumOutside implements MixinForEnum { x }
+//                          ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'MixinForEnum' can't be implemented outside of its library because it's a base mixin.

tests/language/class_modifiers/base/base_mixin_implement_lib.dart

@@ -10,9 +10,10 @@ base mixin BaseMixin {
   int foo = 0;
 }
 
-abstract class A implements BaseMixin {}
+base mixin MixinForEnum {}
 
-class B implements BaseMixin {
-  @override
+abstract base class A implements BaseMixin {}
+
+base class B implements BaseMixin {
   int foo = 1;
 }

tests/language/class_modifiers/base/base_mixin_typedef_with_lib.dart

@@ -4,13 +4,16 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Library declarations used by 
+// base_mixin_typedef_with_outside_test.dart and
+// base_mixin_typedef_with_test.dart.
+
 base mixin BaseMixin {
   int foo = 0;
 }
 
 typedef BaseMixinTypeDef = BaseMixin;
 
-class A with BaseMixinTypeDef {
-  @override
+base class A with BaseMixinTypeDef {
   int foo = 1;
 }

tests/language/class_modifiers/base/base_mixin_typedef_with_outside_test.dart

@@ -12,7 +12,7 @@ import 'base_mixin_typedef_with_lib.dart';
 
 abstract base class AOutside with BaseMixinTypeDef {}
 
-class AOutsideImpl extends AOutside {}
+base class AOutsideImpl extends AOutside {}
 
 base class BOutside with BaseMixinTypeDef {}
 

tests/language/class_modifiers/base/base_mixin_with_lib.dart

@@ -10,6 +10,10 @@ base mixin BaseMixin {
   int foo = 0;
 }
 
-abstract class A with BaseMixin {}
+base mixin MixinForEnum {}
 
-class B with BaseMixin {}
+abstract base class A with BaseMixin {}
+
+base class B with BaseMixin {}
+
+enum EnumInside with MixinForEnum { x }

tests/language/class_modifiers/base/base_mixin_with_not_base_final_error_test.dart

@@ -4,8 +4,8 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
-// Error when mixing in a base mixin where the class mixing it in is not base or
-// final.
+// Error when mixing in a base mixin where the class mixing it in is not base,
+// final, or sealed.
 
 import 'base_mixin_with_lib.dart';
 

tests/language/class_modifiers/base/base_mixin_with_outside_test.dart

@@ -11,11 +11,14 @@ import 'base_mixin_with_lib.dart';
 
 abstract base class AOutside with BaseMixin {}
 
-class AOutsideImpl extends AOutside {}
+base class AOutsideImpl extends AOutside {}
 
 base class BOutside with BaseMixin {}
 
+enum EnumOutside with MixinForEnum { x }
+
 main() {
   Expect.equals(0, AOutsideImpl().foo);
   Expect.equals(0, BOutside().foo);
+  Expect.equals(0, EnumOutside.x.index);
 }

tests/language/class_modifiers/base/base_mixin_with_test.dart

@@ -9,9 +9,10 @@
 import 'package:expect/expect.dart';
 import 'base_mixin_with_lib.dart';
 
-class AImpl extends A {}
+base class AImpl extends A {}
 
 main() {
   Expect.equals(0, AImpl().foo);
   Expect.equals(0, B().foo);
+  Expect.equals(0, EnumInside.x.index);
 }

tests/language/class_modifiers/final/final_class_abstract_construct_error_test.dart

@@ -13,11 +13,11 @@ abstract final class AlsoNotConstructable = Object with M;
 
 main() {
   var error = NotConstructable();
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
+//            ^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INSTANTIATE_ABSTRACT_CLASS
+// [cfe] The class 'NotConstructable' is abstract and can't be instantiated.
   var error2 = AlsoNotConstructable();
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
+//             ^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INSTANTIATE_ABSTRACT_CLASS
+// [cfe] The class 'AlsoNotConstructable' is abstract and can't be instantiated.
 }

tests/language/class_modifiers/final/final_class_base_class_lib.dart

@@ -4,10 +4,13 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Library for having a `final` subtype on a `base` superclass outside its
+// library.
+
 base class BaseClass {
   int foo = 0;
 }
 
 base mixin BaseMixin {
-  int foo = 0; 
+  int foo = 0;
 }

tests/language/class_modifiers/final/final_class_extend_lib.dart

@@ -11,6 +11,8 @@ final class FinalClass {
   int foo = 0;
 }
 
-abstract class A extends FinalClass {}
+abstract final class A extends FinalClass {}
 
-class B extends FinalClass {}
+final class AImpl extends A {}
+
+final class B extends FinalClass {}

tests/language/class_modifiers/final/final_class_extend_outside_error_test.dart

@@ -8,13 +8,13 @@
 
 import 'final_class_extend_lib.dart';
 
-abstract class AOutside extends FinalClass {}
-//                              ^^^^^^^^^^
+abstract final class AOutside extends FinalClass {}
+//                                    ^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'FinalClass' can't be extended outside of its library because it's a final class.
 
-class BOutside extends FinalClass {
-//                     ^^^^^^^^^^
+final class BOutside extends FinalClass {
+//                           ^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'FinalClass' can't be extended outside of its library because it's a final class.
 }

tests/language/class_modifiers/final/final_class_extend_test.dart

@@ -10,8 +10,6 @@
 import 'package:expect/expect.dart';
 import 'final_class_extend_lib.dart';
 
-class AImpl extends A {}
-
 main() {
   Expect.equals(0, AImpl().foo);
   Expect.equals(0, B().foo);

tests/language/class_modifiers/final/final_class_final_subtype_test.dart

@@ -13,7 +13,6 @@ final class FinalClass {
 }
 final class A extends FinalClass {}
 final class B implements FinalClass {
-  @override
   int foo = 1;
 }
 
@@ -21,18 +20,17 @@ final mixin FinalMixin {
   int foo = 0;
 }
 final class C implements FinalMixin {
-  @override
   int foo = 1;
 }
 final class AMixin with FinalMixin {}
 final class BMixin = Object with FinalMixin;
 
 // Used for trivial runtime tests of the final subtypes.
-class AConcrete extends A {}
-class BConcrete extends B {}
-class CConcrete extends C {}
-class AMixinConcrete extends AMixin {}
-class BMixinConcrete extends BMixin {}
+final class AConcrete extends A {}
+final class BConcrete extends B {}
+final class CConcrete extends C {}
+final class AMixinConcrete extends AMixin {}
+final class BMixinConcrete extends BMixin {}
 
 main() {
   Expect.equals(0, AConcrete().foo);

tests/language/class_modifiers/final/final_class_implement_lib.dart

@@ -10,9 +10,16 @@ final class FinalClass {
   int foo = 0;
 }
 
-abstract class A implements FinalClass {}
+final class ClassForEnum {}
 
-class B implements FinalClass {
-  @override
+abstract final class A implements FinalClass {}
+
+final class AImpl implements A {
   int foo = 1;
 }
+
+final class B implements FinalClass {
+  int foo = 1;
+}
+
+enum EnumInside implements ClassForEnum { x }

tests/language/class_modifiers/final/final_class_implement_outside_error_test.dart

@@ -8,15 +8,19 @@
 
 import 'final_class_implement_lib.dart';
 
-abstract class AOutside implements FinalClass {}
-//                                 ^^^^^^^^^^
+abstract final class AOutside implements FinalClass {}
+//                                       ^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'FinalClass' can't be implemented outside of its library because it's a final class.
 
-class BOutside implements FinalClass {
-//                        ^^^^^^^^^^
+final class BOutside implements FinalClass {
+//                              ^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'FinalClass' can't be implemented outside of its library because it's a final class.
-  @override
   int foo = 1;
 }
+
+enum EnumOutside implements ClassForEnum { x }
+//                          ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'ClassForEnum' can't be implemented outside of its library because it's a final class.

tests/language/class_modifiers/final/final_class_implement_test.dart

@@ -10,12 +10,8 @@
 import 'package:expect/expect.dart';
 import 'final_class_implement_lib.dart';
 
-class AImpl implements A {
-  @override
-  int foo = 1;
-}
-
 main() {
   Expect.equals(1, AImpl().foo);
   Expect.equals(1, B().foo);
+  Expect.equals(0, EnumInside.x.index);
 }

tests/language/class_modifiers/final/final_class_inside_not_base_final_sealed_error_test.dart

@@ -0,0 +1,36 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers
+
+// Error when subtyping a final class where the subtype is not base, final or
+// sealed.
+
+final class FinalClass {}
+final mixin FinalMixin {}
+
+class Extends extends FinalClass {}
+// ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+class Implements implements FinalClass {}
+// ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+mixin MixinImplements implements FinalMixin {}
+// ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+class With with FinalMixin {}
+// ^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+mixin On on FinalClass {}
+// ^
+// [analyzer] unspecified
+// [cfe] unspecified

tests/language/class_modifiers/final/final_class_mixin_on_lib.dart

@@ -1,17 +0,0 @@
-// 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.
-
-// SharedOptions=--enable-experiment=class-modifiers
-
-final class FinalClass {}
-
-abstract class A extends FinalClass {}
-
-class B extends FinalClass {}
-
-final mixin FinalMixin {}
-
-class C extends FinalClass with FinalMixin {}
-
-class D with FinalMixin {}

tests/language/class_modifiers/final/final_class_mixin_on_test.dart

@@ -4,28 +4,38 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
-// Allow a final type to appear in the "on" clause of a mixin declaration in
-// another library.
+// Allow a final type to appear in the "on" clause of a mixin declaration.
 
 import 'package:expect/expect.dart';
-import 'final_class_mixin_on_lib.dart';
 
-mixin MA on FinalClass {}
-mixin MB on FinalClass {}
+final class FinalClass {}
 
-class ConcreteA extends A with MA, MB {
+abstract final class A extends FinalClass {}
+
+final class B extends FinalClass {}
+
+final mixin FinalMixin {}
+
+final class C extends FinalClass with FinalMixin {}
+
+final class D with FinalMixin {}
+
+final mixin MA on FinalClass {}
+final mixin MB on FinalClass {}
+
+final class ConcreteA extends A with MA, MB {
   int foo = 0;
 }
 
-mixin MC on FinalClass, FinalMixin {}
+final mixin MC on FinalClass, FinalMixin {}
 
-class ConcreteC extends C with MC {
+final class ConcreteC extends C with MC {
   int foo = 0;
 }
 
-mixin MCSingular on FinalMixin {}
+final mixin MCSingular on FinalMixin {}
 
-class ConcreteD extends D with MCSingular {
+final class ConcreteD extends D with MCSingular {
   int foo = 0;
 }
 

tests/language/class_modifiers/final/final_class_part_lib.dart

@@ -4,11 +4,12 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Library declarations used by final_class_part_test.dart.
+
 part of 'final_class_part_test.dart';
 
-class A extends FinalClass {}
+final class A extends FinalClass {}
 
-class B implements FinalClass {
-  @override
+final class B implements FinalClass {
   int foo = 1;
 }

tests/language/class_modifiers/final/final_class_subtype_base_final_or_sealed_test.dart

@@ -0,0 +1,74 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers,sealed-class
+
+// Allow final mixins/classes to be subtyped by base, final, or sealed
+// classes/mixins and produce no error.
+
+import 'package:expect/expect.dart';
+
+final class FinalClass {
+  int foo = 0;
+}
+final mixin FinalMixin {
+  int foo = 0;
+}
+
+base class BaseExtends extends FinalClass {}
+final class FinalExtends extends FinalClass {}
+sealed class SealedExtends extends FinalClass {}
+final class SealedExtendsImpl extends SealedExtends {}
+
+base class BaseImplements implements FinalClass {
+  int foo = 0;
+}
+final class FinalImplements implements FinalClass {
+  int foo = 0;
+}
+sealed class SealedImplements implements FinalClass {
+  int foo = 0;
+}
+final class SealedImplementsImpl extends SealedImplements {}
+
+base mixin BaseMixinImplements implements FinalMixin {}
+final mixin FinalMixinImplements implements FinalMixin {}
+sealed mixin SealedMixinImplements implements FinalMixin {}
+
+final class ImplementsImpl implements BaseMixinImplements, FinalMixinImplements, SealedMixinImplements {
+  int foo = 0;
+}
+
+base class BaseWith with FinalMixin {}
+final class FinalWith with FinalMixin {}
+sealed class SealedWith with FinalMixin {}
+final class SealedWithImpl extends SealedWith {}
+
+base mixin BaseOn on FinalClass {}
+final mixin FinalOn on FinalClass {}
+sealed mixin SealedOn on FinalClass {}
+
+final class OnImpl implements BaseOn, FinalOn, SealedOn {
+  int foo = 0;
+}
+
+base mixin MixinForEnum {}
+enum EnumWith with MixinForEnum { x }
+enum EnumImplements implements MixinForEnum { x }
+
+main() {
+  Expect.equals(0, BaseExtends().foo);
+  Expect.equals(0, FinalExtends().foo);
+  Expect.equals(0, SealedExtendsImpl().foo);
+  Expect.equals(0, BaseImplements().foo);
+  Expect.equals(0, FinalImplements().foo);
+  Expect.equals(0, SealedImplementsImpl().foo);
+  Expect.equals(0, ImplementsImpl().foo);
+  Expect.equals(0, BaseWith().foo);
+  Expect.equals(0, FinalWith().foo);
+  Expect.equals(0, SealedWithImpl().foo);
+  Expect.equals(0, OnImpl().foo);
+  Expect.equals(0, EnumWith.x.index);
+  Expect.equals(0, EnumImplements.x.index);
+}

tests/language/class_modifiers/final/final_class_typedef_lib.dart

@@ -4,15 +4,18 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by 
+// final_class_typedef_subtype_error_test.dart and
+// final_class_typedef_test.dart.
+
 final class FinalClass {
   int foo = 0;
 }
 
 typedef FinalClassTypeDef = FinalClass;
 
-class A extends FinalClassTypeDef {}
+final class A extends FinalClassTypeDef {}
 
-class B implements FinalClassTypeDef {
-  @override
+final class B implements FinalClassTypeDef {
   int foo = 1;
 }

tests/language/class_modifiers/final/final_class_typedef_outside_of_library_lib.dart

@@ -4,14 +4,17 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// final_class_typedef_outside_of_library_test.dart.
+
 import 'final_class_typedef_outside_of_library_lib2.dart';
 
 final class FinalClass {
   int foo = 0;
 }
 
-class A extends ATypeDef {}
+final class A extends ATypeDef {}
 
-class B implements ATypeDef {
+final class B implements ATypeDef {
   int foo = 1;
 }

tests/language/class_modifiers/final/final_class_typedef_outside_of_library_lib2.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// final_class_typedef_outside_of_library_test.dart.
+
 import 'final_class_typedef_outside_of_library_lib.dart';
 
 typedef ATypeDef = FinalClass;

tests/language/class_modifiers/final/final_class_typedef_subtype_error_test.dart

@@ -9,15 +9,14 @@
 
 import 'final_class_typedef_lib.dart';
 
-class ATypeDef extends FinalClassTypeDef {}
-//                     ^^^^^^^^^^^^^^^^^
+final class ATypeDef extends FinalClassTypeDef {}
+//                           ^^^^^^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'FinalClass' can't be extended outside of its library because it's a final class.
 
-class BTypeDef implements FinalClassTypeDef {
-//                        ^^^^^^^^^^^^^^^^^
+final class BTypeDef implements FinalClassTypeDef {
+//                              ^^^^^^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'FinalClass' can't be implemented outside of its library because it's a final class.
-  @override
   int foo = 1;
 }

tests/language/class_modifiers/final/final_class_typedef_used_outside_error_test.dart

@@ -12,13 +12,13 @@ import 'final_class_typedef_used_outside_lib.dart';
 
 typedef ATypeDef = FinalClass;
 
-class A extends ATypeDef {}
-//              ^^^^^^^^
+final class A extends ATypeDef {}
+//                    ^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'FinalClass' can't be extended outside of its library because it's a final class.
 
-class B implements ATypeDef {
-//                 ^^^^^^^^
+final class B implements ATypeDef {
+//                       ^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'FinalClass' can't be implemented outside of its library because it's a final class.
   int foo = 1;

tests/language/class_modifiers/final/final_class_typedef_used_outside_lib.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// final_class_typedef_used_outside_error_test.dart.
+
 final class FinalClass {
   int foo = 0;
 }

tests/language/class_modifiers/final/final_mixin_implement_lib.dart

@@ -10,9 +10,16 @@ final mixin FinalMixin {
   int foo = 0;
 }
 
-abstract class A implements FinalMixin {}
+final mixin MixinForEnum {}
 
-class B implements FinalMixin {
-  @override
+abstract final class A implements FinalMixin {}
+
+final class AImpl implements A {
   int foo = 1;
 }
+
+final class B implements FinalMixin {
+  int foo = 1;
+}
+
+enum EnumInside implements MixinForEnum { x }

tests/language/class_modifiers/final/final_mixin_implement_outside_error_test.dart

@@ -8,15 +8,19 @@
 
 import 'final_mixin_implement_lib.dart';
 
-abstract class AOutside implements FinalMixin {}
-//                                 ^^^^^^^^^^
+abstract final class AOutside implements FinalMixin {}
+//                                       ^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The mixin 'FinalMixin' can't be implemented outside of its library because it's a final mixin.
 
-class BOutside implements FinalMixin {
-//                        ^^^^^^^^^^
+final class BOutside implements FinalMixin {
+//                              ^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The mixin 'FinalMixin' can't be implemented outside of its library because it's a final mixin.
-  @override
   int foo = 1;
 }
+
+enum EnumOutside implements MixinForEnum { x }
+//                          ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'MixinForEnum' can't be implemented outside of its library because it's a final mixin.

tests/language/class_modifiers/final/final_mixin_implement_test.dart

@@ -9,12 +9,8 @@
 import 'package:expect/expect.dart';
 import 'final_mixin_implement_lib.dart';
 
-class AImpl implements A {
-  @override
-  int foo = 1;
-}
-
 main() {
   Expect.equals(1, AImpl().foo);
   Expect.equals(1, B().foo);
+  Expect.equals(0, EnumInside.x.index);
 }

tests/language/class_modifiers/final/final_mixin_typedef_with_lib.dart

@@ -4,13 +4,15 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// final_mixin_typedef_with_outside_error_test.dart.
+
 final mixin FinalMixin {
   int foo = 0;
 }
 
 typedef FinalMixinTypeDef = FinalMixin;
 
-class A with FinalMixinTypeDef {
-  @override
+final class A with FinalMixinTypeDef {
   int foo = 1;
 }

tests/language/class_modifiers/final/final_mixin_typedef_with_outside_error_test.dart

@@ -8,12 +8,12 @@
 
 import 'final_mixin_typedef_with_lib.dart';
 
-abstract class AOutside with FinalMixinTypeDef {}
-//                           ^^^^^^^^^^^^^^^^^
+abstract final class AOutside with FinalMixinTypeDef {}
+//                                 ^^^^^^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The mixin 'FinalMixin' can't be mixed-in outside of its library because it's a final mixin.
 
-class BOutside with FinalMixinTypeDef {}
-//                  ^^^^^^^^^^^^^^^^^
+final class BOutside with FinalMixinTypeDef {}
+//                        ^^^^^^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The mixin 'FinalMixin' can't be mixed-in outside of its library because it's a final mixin.

tests/language/class_modifiers/final/final_mixin_with_lib.dart

@@ -10,6 +10,12 @@ final mixin FinalMixin {
   int foo = 0;
 }
 
-abstract class A with FinalMixin {}
+final mixin MixinForEnum {}
 
-class B with FinalMixin {}
+abstract final class A with FinalMixin {}
+
+final class AImpl extends A {}
+
+final class B with FinalMixin {}
+
+enum EnumInside with MixinForEnum { x }

tests/language/class_modifiers/final/final_mixin_with_outside_error_test.dart

@@ -8,12 +8,17 @@
 
 import 'final_mixin_with_lib.dart';
 
-abstract class AOutside with FinalMixin {}
-//                           ^^^^^^^^^^
+abstract final class AOutside with FinalMixin {}
+//                                 ^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
-// [cfe] unspecified
+// [cfe] The mixin 'FinalMixin' can't be mixed-in outside of its library because it's a final mixin.
 
-class BOutside with FinalMixin {}
-//                  ^^^^^^^^^^
+final class BOutside with FinalMixin {}
+//                        ^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
-// [cfe] unspecified
+// [cfe] The mixin 'FinalMixin' can't be mixed-in outside of its library because it's a final mixin.
+
+enum EnumOutside with MixinForEnum { x }
+//                    ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'MixinForEnum' can't be mixed-in outside of its library because it's a final mixin.

tests/language/class_modifiers/final/final_mixin_with_test.dart

@@ -9,9 +9,8 @@
 import 'package:expect/expect.dart';
 import 'final_mixin_with_lib.dart';
 
-class AImpl extends A {}
-
 main() {
   Expect.equals(0, AImpl().foo);
   Expect.equals(0, B().foo);
+  Expect.equals(0, EnumInside.x.index);
 }

tests/language/class_modifiers/interface/interface_class_abstract_construct_error_test.dart

@@ -13,11 +13,11 @@ abstract interface class AlsoNotConstructable = Object with M;
 
 main() {
   var error = NotConstructable();
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
+//            ^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INSTANTIATE_ABSTRACT_CLASS
+// [cfe] The class 'NotConstructable' is abstract and can't be instantiated.
   var error2 = AlsoNotConstructable();
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
+//             ^^^^^^^^^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INSTANTIATE_ABSTRACT_CLASS
+// [cfe] The class 'AlsoNotConstructable' is abstract and can't be instantiated.
 }

tests/language/class_modifiers/interface/interface_class_implement_lib.dart

@@ -11,9 +11,12 @@ interface class InterfaceClass {
   int foo = 0;
 }
 
+interface class ClassForEnum {}
+
 abstract class A implements InterfaceClass {}
 
 class B implements InterfaceClass {
-  @override
   int foo = 1;
 }
+
+enum EnumInside implements ClassForEnum { x }

tests/language/class_modifiers/interface/interface_class_implement_outside_test.dart

@@ -13,19 +13,20 @@ import 'interface_class_implement_lib.dart';
 abstract class AOutside implements InterfaceClass {}
 
 class AOutsideImpl implements AOutside {
-  @override
   int foo = 1;
 }
 
 class DoesNotPreventExtension extends AOutsideImpl {}
 
 class BOutside implements InterfaceClass {
-  @override
   int foo = 1;
 }
 
+enum EnumOutside implements ClassForEnum { x }
+
 main() {
   Expect.equals(1, AOutsideImpl().foo);
   Expect.equals(1, DoesNotPreventExtension().foo);
   Expect.equals(1, BOutside().foo);
+  Expect.equals(0, EnumOutside.x.index);
 }

tests/language/class_modifiers/interface/interface_class_implement_test.dart

@@ -11,11 +11,11 @@ import 'package:expect/expect.dart';
 import 'interface_class_implement_lib.dart';
 
 class AImpl implements A {
-  @override
   int foo = 1;
 }
 
 main() {
   Expect.equals(1, AImpl().foo);
   Expect.equals(1, B().foo);
+  Expect.equals(0, EnumInside.x.index);
 }

tests/language/class_modifiers/interface/interface_class_interface_subtype_test.dart

@@ -13,7 +13,6 @@ interface class InterfaceClass {
 }
 interface class A extends InterfaceClass {}
 interface class B implements InterfaceClass {
-  @override
   int foo = 1;
 }
 
@@ -34,4 +33,4 @@ main() {
   Expect.equals(1, BConcrete().foo);
   Expect.equals(0, AMixinConcrete().foo);
   Expect.equals(0, BMixinConcrete().foo);
-}
+}

tests/language/class_modifiers/interface/interface_class_mixin_on_lib.dart

@@ -4,6 +4,8 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by interface_class_mixin_on_test.dart.
+
 interface class InterfaceClass {}
 
 abstract class A extends InterfaceClass {}

tests/language/class_modifiers/interface/interface_class_part_lib.dart

@@ -4,11 +4,12 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by interface_class_part_test.dart.
+
 part of 'interface_class_part_test.dart';
 
 class A extends InterfaceClass {}
 
 class B implements InterfaceClass {
-  @override
   int foo = 1;
 }

tests/language/class_modifiers/interface/interface_class_syntax_disabled_error_test.dart

@@ -1,22 +0,0 @@
-// 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.
-
-// Make sure errors are emitted when trying to use interface classes without
-// the `class-modifiers` experiment enabled.
-
-interface class InterfaceClass {}
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
-
-mixin M {}
-interface class InterfaceClassTypeAlias = Object with M;
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified
-
-interface mixin InterfaceMixin {}
-// ^
-// [analyzer] unspecified
-// [cfe] unspecified

tests/language/class_modifiers/interface/interface_class_typedef_lib.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other library declarations used by
+// interface_class_typedef_extend_error_test.dart.
+
 interface class InterfaceClass {
   int foo = 0;
 }
@@ -13,6 +16,5 @@ typedef InterfaceClassTypeDef = InterfaceClass;
 class A extends InterfaceClassTypeDef {}
 
 class B implements InterfaceClassTypeDef {
-  @override
   int foo = 1;
 }

tests/language/class_modifiers/interface/interface_class_typedef_outside_of_library_lib.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// interface_class_typedef_outside_of_library_test.dart.
+
 import 'interface_class_typedef_outside_of_library_lib2.dart';
 
 interface class InterfaceClass {

tests/language/class_modifiers/interface/interface_class_typedef_outside_of_library_lib2.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// interface_class_typedef_outside_of_library_test.dart.
+
 import "interface_class_typedef_outside_of_library_lib.dart";
 
 typedef ATypeDef = InterfaceClass;

tests/language/class_modifiers/interface/interface_class_typedef_used_outside_lib.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// interface_class_typedef_used_outside_error_test.dart.
+
 interface class InterfaceClass {
   int foo = 0;
 }

tests/language/class_modifiers/interface/interface_mixin_implement_lib.dart

@@ -11,9 +11,10 @@ interface mixin InterfaceMixin {
   int foo = 0;
 }
 
+interface mixin MixinForEnum {}
+
 abstract class A implements InterfaceMixin {}
 
 class B implements InterfaceMixin {
-  @override
   int foo = 1;
 }

tests/language/class_modifiers/interface/interface_mixin_implement_outside_test.dart

@@ -13,16 +13,17 @@ import 'interface_mixin_implement_lib.dart';
 abstract class AOutside implements InterfaceMixin {}
 
 class AOutsideImpl implements AOutside {
-  @override
   int foo = 1;
 }
 
 class BOutside implements InterfaceMixin {
-  @override
   int foo = 1;
 }
 
+enum EnumOutside implements MixinForEnum { x }
+
 main() {
   Expect.equals(1, AOutsideImpl().foo);
   Expect.equals(1, BOutside().foo);
+  Expect.equals(0, EnumOutside.x.index);
 }

tests/language/class_modifiers/interface/interface_mixin_typedef_with_lib.dart

@@ -4,6 +4,10 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// interface_mixin_typedef_with_outside_error_test.dart and
+// interface_mixin_typedef_with_test.dart.
+
 interface mixin InterfaceMixin {
   int foo = 0;
 }
@@ -11,6 +15,5 @@ interface mixin InterfaceMixin {
 typedef InterfaceMixinTypeDef = InterfaceMixin;
 
 class A with InterfaceMixinTypeDef {
-  @override
   int foo = 1;
 }

tests/language/class_modifiers/interface/interface_mixin_with_lib.dart

@@ -10,6 +10,10 @@ interface mixin InterfaceMixin {
   int foo = 0;
 }
 
+interface mixin MixinForEnum {}
+
 abstract class A with InterfaceMixin {}
 
 class B with InterfaceMixin {}
+
+enum EnumInside implements MixinForEnum { x }

tests/language/class_modifiers/interface/interface_mixin_with_outside_error_test.dart

@@ -17,3 +17,8 @@ class BOutside with InterfaceMixin {}
 //                  ^^^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The mixin 'InterfaceMixin' can't be mixed-in outside of its library because it's an interface mixin.
+
+enum EnumOutside with MixinForEnum { x }
+//                    ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The mixin 'MixinForEnum' can't be mixed-in outside of its library because it's an interface mixin.

tests/language/class_modifiers/interface/interface_mixin_with_test.dart

@@ -14,4 +14,5 @@ class AImpl extends A {}
 main() {
   Expect.equals(0, AImpl().foo);
   Expect.equals(0, B().foo);
+  Expect.equals(0, EnumInside.x.index);
 }

tests/language/class_modifiers/legacy/base_class_syntax_disabled_error_test.dart

@@ -0,0 +1,25 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers
+// @dart = 2.19
+
+// Make sure errors are emitted when trying to use base classes without
+// the `class-modifiers` experiment enabled.
+
+base class BaseClass {}
+// [error column 1, length 4]
+// [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
+// [cfe] The 'class-modifiers' language feature is disabled for this library.
+
+mixin M {}
+base class BaseClassTypeAlias = Object with M;
+// [error column 1, length 4]
+// [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
+// [cfe] The 'class-modifiers' language feature is disabled for this library.
+
+base mixin BaseMixin {}
+// [error column 1, length 4]
+// [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
+// [cfe] The 'class-modifiers' language feature is disabled for this library.

tests/language/class_modifiers/legacy/final_class_syntax_disabled_error_test.dart

@@ -1,22 +1,25 @@
-// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2023, 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.
 
+// SharedOptions=--enable-experiment=class-modifiers
+// @dart = 2.19
+
 // Make sure errors are emitted when trying to use final classes without
 // the `class-modifiers` experiment enabled.
 
 final class FinalClass {}
 // [error column 1, length 5]
 // [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
-// [cfe] This requires the experimental 'class-modifiers' language feature to be enabled.
+// [cfe] The 'class-modifiers' language feature is disabled for this library.
 
 mixin M {}
 final class FinalClassTypeAlias = Object with M;
 // [error column 1, length 5]
 // [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
-// [cfe] This requires the experimental 'class-modifiers' language feature to be enabled.
+// [cfe] The 'class-modifiers' language feature is disabled for this library.
 
 final mixin FinalMixin {}
 // [error column 1, length 5]
 // [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
-// [cfe] This requires the experimental 'class-modifiers' language feature to be enabled.
+// [cfe] The 'class-modifiers' language feature is disabled for this library.

tests/language/class_modifiers/legacy/interface_class_syntax_disabled_error_test.dart

@@ -0,0 +1,25 @@
+// Copyright (c) 2023, 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.
+
+// SharedOptions=--enable-experiment=class-modifiers
+// @dart = 2.19
+
+// Make sure errors are emitted when trying to use interface classes without
+// the `class-modifiers` experiment enabled.
+
+interface class InterfaceClass {}
+// [error column 1, length 9]
+// [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
+// [cfe] The 'class-modifiers' language feature is disabled for this library.
+
+mixin M {}
+interface class InterfaceClassTypeAlias = Object with M;
+// [error column 1, length 9]
+// [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
+// [cfe] The 'class-modifiers' language feature is disabled for this library.
+
+interface mixin InterfaceMixin {}
+// [error column 1, length 9]
+// [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
+// [cfe] The 'class-modifiers' language feature is disabled for this library.

tests/language/class_modifiers/legacy/mixin_class_syntax_disabled_error_test.dart

@@ -2,21 +2,24 @@
 // 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.
 
+// SharedOptions=--enable-experiment=class-modifiers
+// @dart = 2.19
+
 // Make sure errors are emitted when trying to use mixin classes without
 // the `class-modifiers` experiment enabled.
 
 mixin class MixinClass {}
 // [error column 1, length 5]
 // [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
-// [cfe] This requires the experimental 'class-modifiers' language feature to be enabled.
+// [cfe] The 'class-modifiers' language feature is disabled for this library.
 
 abstract mixin class AbstractMixinClass {}
 //       ^^^^^
 // [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
-// [cfe] This requires the experimental 'class-modifiers' language feature to be enabled.
+// [cfe] The 'class-modifiers' language feature is disabled for this library.
 
 mixin M {}
 mixin class NamedMixinClassApplication = Object with M;
 // [error column 1, length 5]
 // [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
-// [cfe] This requires the experimental 'class-modifiers' language feature to be enabled.
+// [cfe] The 'class-modifiers' language feature is disabled for this library.

tests/language/class_modifiers/mixin/mixin_class_no_modifier_lib.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// mixin_class_no_modifier_outside_library_error_test.dart.
+
 class Class {
   int foo = 0;
 }

tests/language/class_modifiers/mixin/mixin_class_no_modifier_old_version_lib.dart

@@ -4,6 +4,9 @@
 
 // @dart = 2.19
 
+// Testing usage of classes as mixins with no 'mixin' modifier in an older
+// version.
+
 class Class {
   int foo = 0;
 }

tests/language/class_modifiers/mixin/mixin_class_no_modifier_old_version_superclass_not_object_error_test.dart

@@ -13,7 +13,6 @@ class SubclassNotObject with NonObjectSuperclassClass {
 //                           ^^^^^^^^^^^^^^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.MIXIN_INHERITS_FROM_NOT_OBJECT
 // [cfe] The class 'NonObjectSuperclassClass' can't be used as a mixin because it extends a class other than 'Object'.
-  @override
   int foo = 1;
 }
 

tests/language/class_modifiers/mixin/mixin_class_part_lib.dart

@@ -4,6 +4,8 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by mixin_class_part_test.dart.
+
 part of 'mixin_class_part_test.dart';
 
 class A with MixinClass {}

tests/language/class_modifiers/mixin/mixin_class_syntax_error_test.dart

@@ -79,3 +79,10 @@ mixin typedef T = String;
 // [analyzer] SYNTACTIC_ERROR.MISSING_IDENTIFIER
 // [cfe] A mixin declaration must have a body, even if it is empty.
 // [cfe] Expected an identifier, but got 'typedef'.
+
+mixin extension StringExtension on String {}
+//    ^^^^^^^^^
+// [analyzer] SYNTACTIC_ERROR.EXPECTED_BODY
+// [analyzer] SYNTACTIC_ERROR.MISSING_IDENTIFIER
+// [cfe] A mixin declaration must have a body, even if it is empty.
+// [cfe] Expected an identifier, but got 'extension'.

tests/language/class_modifiers/mixin/mixin_class_typedef_lib.dart

@@ -4,6 +4,8 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by mixin_class_typedef_test.dart.
+
 mixin class MixinClass {
   int foo = 0;
 }

tests/language/class_modifiers/mixin/mixin_class_typedef_outside_of_library_lib.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// mixin_class_typedef_outside_of_library_test.dart.
+
 import 'mixin_class_typedef_outside_of_library_lib2.dart';
 
 mixin class MixinClass {

tests/language/class_modifiers/mixin/mixin_class_typedef_outside_of_library_lib2.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// mixin_class_typedef_outside_of_library_test.dart.
+
 import 'mixin_class_typedef_outside_of_library_lib.dart';
 
 typedef ATypeDef = MixinClass;

tests/language/class_modifiers/mixin/mixin_class_typedef_used_outside_lib.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=class-modifiers
 
+// Other-library declarations used by
+// mixin_class_typedef_used_outside_test.dart.
+
 mixin class MixinClass {
   int foo = 0;
 }

tests/language/sealed_class/sealed_class_as_mixin_error_test.dart

@@ -21,10 +21,7 @@ class OutsideB with SealedClass {
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'SealedClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
 // [cfe] The mixin 'SealedClass' can't be mixed in outside of its library because it's a sealed mixin.
-  @override
   int foo = 2;
-
-  @override
   int bar(int value) => value;
 }
 

tests/language/sealed_class/sealed_class_as_mixin_lib.dart

@@ -4,6 +4,8 @@
 
 // SharedOptions=--enable-experiment=sealed-class,class-modifiers
 
+// Other-library declarations used by sealed_class_as_mixin_error_test.dart.
+
 sealed class SealedClass {
   int nonAbstractFoo = 0;
   abstract int foo;

tests/language/sealed_class/sealed_class_extend_error_test.dart

@@ -17,9 +17,6 @@ class OutsideB extends SealedClass {
 //                     ^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'SealedClass' can't be extended, implemented, or mixed in outside of its library because it's a sealed class.
-  @override
   int foo = 2;
-
-  @override
   int bar(int value) => value;
 }

tests/language/sealed_class/sealed_class_extend_lib.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=sealed-class
 
+// Other-library declarations used by sealed_class_extend_test.dart and
+// sealed_class_extend_error_test.dart.
+
 sealed class SealedClass {
   int nonAbstractFoo = 0;
   abstract int foo;
@@ -14,20 +17,12 @@ sealed class SealedClass {
 abstract class A extends SealedClass {}
 
 class AImpl extends A {
-  @override
   int foo = 1;
-
-  @override
   int bar(int value) => value + 1;
 }
 
 class B extends SealedClass {
-  @override
   int nonAbstractFoo = 100;
-
-  @override
   int foo = 2;
-
-  @override
   int bar(int value) => value;
 }

tests/language/sealed_class/sealed_class_extend_test.dart

@@ -11,24 +11,14 @@ import "package:expect/expect.dart";
 import 'sealed_class_extend_lib.dart';
 
 class AExtends extends A {
-  @override
   int foo = 0;
-
-  @override
   int bar(int value) => value;
 }
 
 class AImplements implements A {
-  @override
   int nonAbstractFoo = 0;
-
-  @override
   int foo = 0;
-
-  @override
   int bar(int value) => value;
-
-  @override
   int nonAbstractBar(int value) => value;
 }
 

tests/language/sealed_class/sealed_class_implement_error_test.dart

@@ -17,16 +17,9 @@ class OutsideB implements SealedClass {
 //                        ^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'SealedClass' can't be extended, implemented, or mixed in outside of its library because it's a sealed class.
-  @override
   int nonAbstractFoo = 2;
-
-  @override
   int foo = 2;
-
-  @override
   int nonAbstractBar(int value) => value;
-
-  @override
   int bar(int value) => value;
 }
 
@@ -34,3 +27,8 @@ mixin OutsideMixin implements SealedClass {}
 //                            ^^^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
 // [cfe] The class 'SealedClass' can't be extended, implemented, or mixed in outside of its library because it's a sealed class.
+
+enum EnumOutside implements ClassForEnum { x }
+//                          ^^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.INVALID_USE_OF_TYPE_OUTSIDE_LIBRARY
+// [cfe] The class 'ClassForEnum' can't be extended, implemented, or mixed in outside of its library because it's a sealed class.

tests/language/sealed_class/sealed_class_implement_lib.dart

@@ -4,6 +4,9 @@
 
 // SharedOptions=--enable-experiment=sealed-class
 
+// Other-library declarations used by sealed_class_implement_test.dart and
+// sealed_class_implement_error_test.dart.
+
 sealed class SealedClass {
   int nonAbstractFoo = 0;
   abstract int foo;
@@ -11,32 +14,22 @@ sealed class SealedClass {
   int bar(int value);
 }
 
+sealed class ClassForEnum {}
+
 abstract class A implements SealedClass {}
 
 class AImpl implements A {
-  @override
   int nonAbstractFoo = 0;
-
-  @override
   int foo = 1;
-
-  @override
   int nonAbstractBar(int value) => value + 100;
-
-  @override
   int bar(int value) => value + 1;
 }
 
 class B implements SealedClass {
-  @override
   int nonAbstractFoo = 100;
-
-  @override
   int foo = 2;
-
-  @override
   int nonAbstractBar(int value) => value + 100;
-
-  @override
   int bar(int value) => value;
 }
+
+enum EnumInside implements ClassForEnum { x }

tests/language/sealed_class/sealed_class_implement_test.dart

@@ -12,30 +12,16 @@ import "package:expect/expect.dart";
 import 'sealed_class_implement_lib.dart';
 
 class AExtends extends A {
-  @override
   int nonAbstractFoo = 0;
-
-  @override
   int foo = 0;
-
-  @override
   int bar(int value) => value;
-
-  @override
   int nonAbstractBar(int value) => value;
 }
 
 class AImplements implements A {
-  @override
   int nonAbstractFoo = 0;
-
-  @override
   int foo = 0;
-
-  @override
   int bar(int value) => value;
-
-  @override
   int nonAbstractBar(int value) => value;
 }
 
@@ -63,4 +49,6 @@ main() {
   Expect.equals(0, aImplements.foo);
   Expect.equals(0, aImplements.bar(0));
   Expect.equals(0, aImplements.nonAbstractBar(0));
+
+  Expect.equals(0, EnumInside.x.index);
 }

tests/language/sealed_class/sealed_class_mixin_on_lib.dart

@@ -4,6 +4,8 @@
 
 // SharedOptions=--enable-experiment=sealed-class
 
+// Other-library declarations used by sealed_class_mixin_on_test.dart.
+
 sealed class SealedClass {}
 
 abstract class A extends SealedClass {}

tests/language/sealed_class/sealed_class_part_lib.dart

@@ -4,11 +4,12 @@
 
 // SharedOptions=--enable-experiment=sealed-class
 
+// Other-library declarations used by sealed_class_part_test.dart.
+
 part of 'sealed_class_part_test.dart';
 
 class A extends SealedClass {}
 
 class B implements SealedClass {
-  @override
   int foo = 1;
 }

tests/language/sealed_class/sealed_class_syntax_disabled_error_test.dart

@@ -2,13 +2,16 @@
 // 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.
 
+// SharedOptions=--enable-experiment=sealed-class
+// @dart = 2.19
+
 // Make sure errors are emitted when trying to use sealed classes without
 // the `sealed` experiment enabled.
 
 sealed class SealedClass {
 // [error column 1, length 6]
 // [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
-// [cfe] This requires the experimental 'sealed-class' language feature to be enabled.
+// [cfe] The 'sealed-class' language feature is disabled for this library.
 //           ^
 // [cfe] The non-abstract class 'SealedClass' is missing implementations for these members:
   int nonAbstractFoo = 0;
@@ -21,34 +24,13 @@ sealed class SealedClass {
 // [analyzer] COMPILE_TIME_ERROR.CONCRETE_CLASS_WITH_ABSTRACT_MEMBER
 }
 
-abstract class A extends SealedClass {}
-
-class AImpl extends A {
-  @override
-  int foo = 1;
-
-  @override
-  int bar(int value) => value + 1;
-}
-
-class B extends SealedClass {
-  @override
-  int nonAbstractFoo = 100;
-
-  @override
-  int foo = 2;
-
-  @override
-  int bar(int value) => value;
-}
-
 mixin M {}
 sealed class SealedClassTypeAlias = Object with M;
 // [error column 1, length 6]
 // [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
-// [cfe] This requires the experimental 'sealed-class' language feature to be enabled.
+// [cfe] The 'sealed-class' language feature is disabled for this library.
 
 sealed mixin SealedMixin {}
 // [error column 1, length 6]
 // [analyzer] SYNTACTIC_ERROR.EXPERIMENT_NOT_ENABLED
-// [cfe] This requires the experimental 'sealed-class' language feature to be enabled.
+// [cfe] The 'sealed-class' language feature is disabled for this library.

tests/language/sealed_class/sealed_class_syntax_error_test.dart

@@ -54,6 +54,7 @@ sealed sealed class SealedDuplicateClass {}
 // [error column 1, length 6]
 // [analyzer] COMPILE_TIME_ERROR.NOT_A_TYPE
 // [cfe] 'sealed' isn't a type.
+// [cfe] Can't use 'sealed' because it is declared more than once.
 //     ^^^^^^
 // [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
 // [cfe] Expected ';' after this.
@@ -63,7 +64,36 @@ class SealedVariable {
     sealed var x = 2;
 //  ^^^^^^
 // [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
+// [cfe] Can't use 'sealed' because it is declared more than once.
 // [cfe] Expected ';' after this.
     return x;
   }
-}
+}
+
+sealed extension StringExtension on String {}
+// [error column 1, length 6]
+// [analyzer] COMPILE_TIME_ERROR.NOT_A_TYPE
+// [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
+// [cfe] 'sealed' isn't a type.
+// [cfe] Expected ';' after this.
+//     ^^^^^^^^^
+// [analyzer] SYNTACTIC_ERROR.MISSING_IDENTIFIER
+// [cfe] Expected an identifier, but got 'extension'.
+
+sealed enum Enum { x }
+// [error column 1, length 6]
+// [analyzer] COMPILE_TIME_ERROR.DUPLICATE_DEFINITION
+// [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
+// [analyzer] SYNTACTIC_ERROR.MISSING_CONST_FINAL_VAR_OR_TYPE
+// [cfe] 'sealed' is already declared in this scope.
+// [cfe] Expected ';' after this.
+// [cfe] Variables must be declared using the keywords 'const', 'final', 'var' or a type name.
+
+sealed typedef EnumTypedef = Enum;
+// [error column 1, length 6]
+// [analyzer] COMPILE_TIME_ERROR.DUPLICATE_DEFINITION
+// [analyzer] SYNTACTIC_ERROR.EXPECTED_TOKEN
+// [analyzer] SYNTACTIC_ERROR.MISSING_CONST_FINAL_VAR_OR_TYPE
+// [cfe] 'sealed' is already declared in this scope.
+// [cfe] Expected ';' after this.
+// [cfe] Variables must be declared using the keywords 'const', 'final', 'var' or a type name.