Improve equivalence testing of unnamed mixin applications.

tests/compiler/dart2js/serialization/test_data.dart

 // Copyright (c) 2016, 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.
 
 library dart2js.serialization_test_data;
 
 const List<Test> TESTS = const <Test>[
   const Test('Empty program', const {
     'main.dart': 'main() {}'
   }),
@@ skipping 309 matching lines @@
   const Test('Serialized symbol literal', const {
     'main.dart': '''
 import 'lib.dart';
 main() => m();
 ''',
   }, preserializedSourceFiles: const {
     'lib.dart': '''
 m() => print(#main);
 ''',
   }),
+
+  const Test('Multiple structurally identical mixins', const {
+    'main.dart': '''
+class S {}
+class M {}
+class C1 extends S with M {}
+class C2 extends S with M {}
+main() {
+  new C1();
+  new C2();
+}
+''',
+  }),
 ];
 
 class Test {
   final String name;
   final Map sourceFiles;
   final Map preserializedSourceFiles;
   final int expectedErrorCount;
   final int expectedWarningCount;
   final int expectedHintCount;
   final int expectedInfoCount;
 
   const Test(
       this.name,
       this.sourceFiles,
       {this.preserializedSourceFiles,
       this.expectedErrorCount: 0,
       this.expectedWarningCount: 0,
       this.expectedHintCount: 0,
       this.expectedInfoCount: 0});
 }