[cfe] Fix bug where we emit an extra error that we don't want for mixin classes.

Would otherwise emit `CantUseClassAsMixin` and then if the supertype is final or interface, would emit `FinalMixinMixedInOutsideOfLibrary` and `InterfaceMixinMixedInOutsideOfLibrary`respectively. These errors together don't make sense so this CL fixes it.

Change-Id: I5fdf4e260b709e08af53cbda0414cff2ef93d3b5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/286951
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

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

@@ -2338,9 +2338,7 @@ severity: $severity
                     .withArguments(mixedInTypeDeclaration.fullNameForErrors),
                 mixedInTypeBuilder.charOffset ?? TreeNode.noOffset,
                 noLength);
-          }
-
-          if (cls.libraryBuilder.origin !=
+          } else if (cls.libraryBuilder.origin !=
               mixedInTypeDeclaration.libraryBuilder.origin) {
             if (mixedInTypeDeclaration.isInterface &&
                 !mayIgnoreClassModifiers(mixedInTypeDeclaration)) {

pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart

@@ -0,0 +1,11 @@
+// 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.
+
+import 'main_lib.dart';
+
+class BaseWith with BaseClass {}
+
+class InterfaceWith with InterfaceClass {}
+
+class FinalWith with FinalClass {}

pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart.strong.expect

@@ -0,0 +1,82 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:7:21: Error: The class 'BaseClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class BaseWith with BaseClass {}
+//                     ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:7:7: Error: The type 'BaseWith' must be 'base', 'final' or 'sealed' because the supertype 'BaseClass' is 'base'.
+// Try adding 'base', 'final', or 'sealed' to the type.
+// class BaseWith with BaseClass {}
+//       ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:9:26: Error: The class 'InterfaceClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class InterfaceWith with InterfaceClass {}
+//                          ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:11:22: Error: The class 'FinalClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class FinalWith with FinalClass {}
+//                      ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:11:7: Error: The type 'FinalWith' must be 'base', 'final' or 'sealed' because the supertype 'FinalClass' is 'final'.
+// Try adding 'base', 'final', or 'sealed' to the type.
+// class FinalWith with FinalClass {}
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+import "main_lib.dart" as mai;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract final class _BaseWith&Object&BaseClass = core::Object with mai::BaseClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_BaseWith&Object&BaseClass
+    : super core::Object::•()
+    ;
+}
+class BaseWith extends self::_BaseWith&Object&BaseClass {
+  synthetic constructor •() → self::BaseWith
+    : super self::_BaseWith&Object&BaseClass::•()
+    ;
+}
+abstract class _InterfaceWith&Object&InterfaceClass = core::Object with mai::InterfaceClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_InterfaceWith&Object&InterfaceClass
+    : super core::Object::•()
+    ;
+}
+class InterfaceWith extends self::_InterfaceWith&Object&InterfaceClass {
+  synthetic constructor •() → self::InterfaceWith
+    : super self::_InterfaceWith&Object&InterfaceClass::•()
+    ;
+}
+abstract final class _FinalWith&Object&FinalClass = core::Object with mai::FinalClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_FinalWith&Object&FinalClass
+    : super core::Object::•()
+    ;
+}
+class FinalWith extends self::_FinalWith&Object&FinalClass {
+  synthetic constructor •() → self::FinalWith
+    : super self::_FinalWith&Object&FinalClass::•()
+    ;
+}
+
+library /*isNonNullableByDefault*/;
+import self as mai;
+import "dart:core" as core;
+
+base class BaseClass extends core::Object {
+  synthetic constructor •() → mai::BaseClass
+    : super core::Object::•()
+    ;
+}
+interface class InterfaceClass extends core::Object {
+  synthetic constructor •() → mai::InterfaceClass
+    : super core::Object::•()
+    ;
+}
+final class FinalClass extends core::Object {
+  synthetic constructor •() → mai::FinalClass
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart.strong.transformed.expect

@@ -0,0 +1,82 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:7:21: Error: The class 'BaseClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class BaseWith with BaseClass {}
+//                     ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:7:7: Error: The type 'BaseWith' must be 'base', 'final' or 'sealed' because the supertype 'BaseClass' is 'base'.
+// Try adding 'base', 'final', or 'sealed' to the type.
+// class BaseWith with BaseClass {}
+//       ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:9:26: Error: The class 'InterfaceClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class InterfaceWith with InterfaceClass {}
+//                          ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:11:22: Error: The class 'FinalClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class FinalWith with FinalClass {}
+//                      ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:11:7: Error: The type 'FinalWith' must be 'base', 'final' or 'sealed' because the supertype 'FinalClass' is 'final'.
+// Try adding 'base', 'final', or 'sealed' to the type.
+// class FinalWith with FinalClass {}
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+import "main_lib.dart" as mai;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract final class _BaseWith&Object&BaseClass extends core::Object implements mai::BaseClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_BaseWith&Object&BaseClass
+    : super core::Object::•()
+    ;
+}
+class BaseWith extends self::_BaseWith&Object&BaseClass {
+  synthetic constructor •() → self::BaseWith
+    : super self::_BaseWith&Object&BaseClass::•()
+    ;
+}
+abstract class _InterfaceWith&Object&InterfaceClass extends core::Object implements mai::InterfaceClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_InterfaceWith&Object&InterfaceClass
+    : super core::Object::•()
+    ;
+}
+class InterfaceWith extends self::_InterfaceWith&Object&InterfaceClass {
+  synthetic constructor •() → self::InterfaceWith
+    : super self::_InterfaceWith&Object&InterfaceClass::•()
+    ;
+}
+abstract final class _FinalWith&Object&FinalClass extends core::Object implements mai::FinalClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_FinalWith&Object&FinalClass
+    : super core::Object::•()
+    ;
+}
+class FinalWith extends self::_FinalWith&Object&FinalClass {
+  synthetic constructor •() → self::FinalWith
+    : super self::_FinalWith&Object&FinalClass::•()
+    ;
+}
+
+library /*isNonNullableByDefault*/;
+import self as mai;
+import "dart:core" as core;
+
+base class BaseClass extends core::Object {
+  synthetic constructor •() → mai::BaseClass
+    : super core::Object::•()
+    ;
+}
+interface class InterfaceClass extends core::Object {
+  synthetic constructor •() → mai::InterfaceClass
+    : super core::Object::•()
+    ;
+}
+final class FinalClass extends core::Object {
+  synthetic constructor •() → mai::FinalClass
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart.textual_outline.expect

@@ -0,0 +1,7 @@
+import 'main_lib.dart';
+
+class BaseWith with BaseClass {}
+
+class InterfaceWith with InterfaceClass {}
+
+class FinalWith with FinalClass {}

pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart.textual_outline_modelled.expect

@@ -0,0 +1,7 @@
+import 'main_lib.dart';
+
+class BaseWith with BaseClass {}
+
+class FinalWith with FinalClass {}
+
+class InterfaceWith with InterfaceClass {}

pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart.weak.expect

@@ -0,0 +1,82 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:7:21: Error: The class 'BaseClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class BaseWith with BaseClass {}
+//                     ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:7:7: Error: The type 'BaseWith' must be 'base', 'final' or 'sealed' because the supertype 'BaseClass' is 'base'.
+// Try adding 'base', 'final', or 'sealed' to the type.
+// class BaseWith with BaseClass {}
+//       ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:9:26: Error: The class 'InterfaceClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class InterfaceWith with InterfaceClass {}
+//                          ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:11:22: Error: The class 'FinalClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class FinalWith with FinalClass {}
+//                      ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:11:7: Error: The type 'FinalWith' must be 'base', 'final' or 'sealed' because the supertype 'FinalClass' is 'final'.
+// Try adding 'base', 'final', or 'sealed' to the type.
+// class FinalWith with FinalClass {}
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+import "main_lib.dart" as mai;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract final class _BaseWith&Object&BaseClass = core::Object with mai::BaseClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_BaseWith&Object&BaseClass
+    : super core::Object::•()
+    ;
+}
+class BaseWith extends self::_BaseWith&Object&BaseClass {
+  synthetic constructor •() → self::BaseWith
+    : super self::_BaseWith&Object&BaseClass::•()
+    ;
+}
+abstract class _InterfaceWith&Object&InterfaceClass = core::Object with mai::InterfaceClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_InterfaceWith&Object&InterfaceClass
+    : super core::Object::•()
+    ;
+}
+class InterfaceWith extends self::_InterfaceWith&Object&InterfaceClass {
+  synthetic constructor •() → self::InterfaceWith
+    : super self::_InterfaceWith&Object&InterfaceClass::•()
+    ;
+}
+abstract final class _FinalWith&Object&FinalClass = core::Object with mai::FinalClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_FinalWith&Object&FinalClass
+    : super core::Object::•()
+    ;
+}
+class FinalWith extends self::_FinalWith&Object&FinalClass {
+  synthetic constructor •() → self::FinalWith
+    : super self::_FinalWith&Object&FinalClass::•()
+    ;
+}
+
+library /*isNonNullableByDefault*/;
+import self as mai;
+import "dart:core" as core;
+
+base class BaseClass extends core::Object {
+  synthetic constructor •() → mai::BaseClass
+    : super core::Object::•()
+    ;
+}
+interface class InterfaceClass extends core::Object {
+  synthetic constructor •() → mai::InterfaceClass
+    : super core::Object::•()
+    ;
+}
+final class FinalClass extends core::Object {
+  synthetic constructor •() → mai::FinalClass
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart.weak.modular.expect

@@ -0,0 +1,62 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:7:21: Error: The class 'BaseClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class BaseWith with BaseClass {}
+//                     ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:7:7: Error: The type 'BaseWith' must be 'base', 'final' or 'sealed' because the supertype 'BaseClass' is 'base'.
+// Try adding 'base', 'final', or 'sealed' to the type.
+// class BaseWith with BaseClass {}
+//       ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:9:26: Error: The class 'InterfaceClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class InterfaceWith with InterfaceClass {}
+//                          ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:11:22: Error: The class 'FinalClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class FinalWith with FinalClass {}
+//                      ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:11:7: Error: The type 'FinalWith' must be 'base', 'final' or 'sealed' because the supertype 'FinalClass' is 'final'.
+// Try adding 'base', 'final', or 'sealed' to the type.
+// class FinalWith with FinalClass {}
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+import "main_lib.dart" as mai;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract final class _BaseWith&Object&BaseClass = core::Object with mai::BaseClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_BaseWith&Object&BaseClass
+    : super core::Object::•()
+    ;
+}
+class BaseWith extends self::_BaseWith&Object&BaseClass {
+  synthetic constructor •() → self::BaseWith
+    : super self::_BaseWith&Object&BaseClass::•()
+    ;
+}
+abstract class _InterfaceWith&Object&InterfaceClass = core::Object with mai::InterfaceClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_InterfaceWith&Object&InterfaceClass
+    : super core::Object::•()
+    ;
+}
+class InterfaceWith extends self::_InterfaceWith&Object&InterfaceClass {
+  synthetic constructor •() → self::InterfaceWith
+    : super self::_InterfaceWith&Object&InterfaceClass::•()
+    ;
+}
+abstract final class _FinalWith&Object&FinalClass = core::Object with mai::FinalClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_FinalWith&Object&FinalClass
+    : super core::Object::•()
+    ;
+}
+class FinalWith extends self::_FinalWith&Object&FinalClass {
+  synthetic constructor •() → self::FinalWith
+    : super self::_FinalWith&Object&FinalClass::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart.weak.outline.expect

@@ -0,0 +1,76 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:7:21: Error: The class 'BaseClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class BaseWith with BaseClass {}
+//                     ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:7:7: Error: The type 'BaseWith' must be 'base', 'final' or 'sealed' because the supertype 'BaseClass' is 'base'.
+// Try adding 'base', 'final', or 'sealed' to the type.
+// class BaseWith with BaseClass {}
+//       ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:9:26: Error: The class 'InterfaceClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class InterfaceWith with InterfaceClass {}
+//                          ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:11:22: Error: The class 'FinalClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class FinalWith with FinalClass {}
+//                      ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:11:7: Error: The type 'FinalWith' must be 'base', 'final' or 'sealed' because the supertype 'FinalClass' is 'final'.
+// Try adding 'base', 'final', or 'sealed' to the type.
+// class FinalWith with FinalClass {}
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+import "main_lib.dart" as mai;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract final class _BaseWith&Object&BaseClass = core::Object with mai::BaseClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_BaseWith&Object&BaseClass
+    : super core::Object::•()
+    ;
+}
+class BaseWith extends self::_BaseWith&Object&BaseClass {
+  synthetic constructor •() → self::BaseWith
+    ;
+}
+abstract class _InterfaceWith&Object&InterfaceClass = core::Object with mai::InterfaceClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_InterfaceWith&Object&InterfaceClass
+    : super core::Object::•()
+    ;
+}
+class InterfaceWith extends self::_InterfaceWith&Object&InterfaceClass {
+  synthetic constructor •() → self::InterfaceWith
+    ;
+}
+abstract final class _FinalWith&Object&FinalClass = core::Object with mai::FinalClass /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_FinalWith&Object&FinalClass
+    : super core::Object::•()
+    ;
+}
+class FinalWith extends self::_FinalWith&Object&FinalClass {
+  synthetic constructor •() → self::FinalWith
+    ;
+}
+
+library /*isNonNullableByDefault*/;
+import self as mai;
+import "dart:core" as core;
+
+base class BaseClass extends core::Object {
+  synthetic constructor •() → mai::BaseClass
+    ;
+}
+interface class InterfaceClass extends core::Object {
+  synthetic constructor •() → mai::InterfaceClass
+    ;
+}
+final class FinalClass extends core::Object {
+  synthetic constructor •() → mai::FinalClass
+    ;
+}

pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart.weak.transformed.expect

@@ -0,0 +1,82 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:7:21: Error: The class 'BaseClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class BaseWith with BaseClass {}
+//                     ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:7:7: Error: The type 'BaseWith' must be 'base', 'final' or 'sealed' because the supertype 'BaseClass' is 'base'.
+// Try adding 'base', 'final', or 'sealed' to the type.
+// class BaseWith with BaseClass {}
+//       ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:9:26: Error: The class 'InterfaceClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class InterfaceWith with InterfaceClass {}
+//                          ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:11:22: Error: The class 'FinalClass' can't be used as a mixin because it isn't a mixin class nor a mixin.
+// class FinalWith with FinalClass {}
+//                      ^
+//
+// pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main.dart:11:7: Error: The type 'FinalWith' must be 'base', 'final' or 'sealed' because the supertype 'FinalClass' is 'final'.
+// Try adding 'base', 'final', or 'sealed' to the type.
+// class FinalWith with FinalClass {}
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+import "main_lib.dart" as mai;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract final class _BaseWith&Object&BaseClass extends core::Object implements mai::BaseClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_BaseWith&Object&BaseClass
+    : super core::Object::•()
+    ;
+}
+class BaseWith extends self::_BaseWith&Object&BaseClass {
+  synthetic constructor •() → self::BaseWith
+    : super self::_BaseWith&Object&BaseClass::•()
+    ;
+}
+abstract class _InterfaceWith&Object&InterfaceClass extends core::Object implements mai::InterfaceClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_InterfaceWith&Object&InterfaceClass
+    : super core::Object::•()
+    ;
+}
+class InterfaceWith extends self::_InterfaceWith&Object&InterfaceClass {
+  synthetic constructor •() → self::InterfaceWith
+    : super self::_InterfaceWith&Object&InterfaceClass::•()
+    ;
+}
+abstract final class _FinalWith&Object&FinalClass extends core::Object implements mai::FinalClass /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_FinalWith&Object&FinalClass
+    : super core::Object::•()
+    ;
+}
+class FinalWith extends self::_FinalWith&Object&FinalClass {
+  synthetic constructor •() → self::FinalWith
+    : super self::_FinalWith&Object&FinalClass::•()
+    ;
+}
+
+library /*isNonNullableByDefault*/;
+import self as mai;
+import "dart:core" as core;
+
+base class BaseClass extends core::Object {
+  synthetic constructor •() → mai::BaseClass
+    : super core::Object::•()
+    ;
+}
+interface class InterfaceClass extends core::Object {
+  synthetic constructor •() → mai::InterfaceClass
+    : super core::Object::•()
+    ;
+}
+final class FinalClass extends core::Object {
+  synthetic constructor •() → mai::FinalClass
+    : super core::Object::•()
+    ;
+}

pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/main_lib.dart

@@ -0,0 +1,9 @@
+// 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.
+
+base class BaseClass {}
+
+interface class InterfaceClass {}
+
+final class FinalClass {}

pkg/front_end/testcases/class_modifiers/mixin/outside_library_modifier/test.options

@@ -0,0 +1 @@
+main_lib.dart