Add mixin support to source mirrors.

tests/compiler/dart2js/mirrors_metadata_test.dart

Index: tests/compiler/dart2js/mirrors_metadata_test.dart
diff --git a/tests/compiler/dart2js/mirrors_metadata_test.dart b/tests/compiler/dart2js/mirrors_metadata_test.dart
index 1e3307cc4cff67ba5b89443fd7a60c95b5fd9266..66c2e416ae070377745131e7860bf38d3671344c 100644
--- a/tests/compiler/dart2js/mirrors_metadata_test.dart
+++ b/tests/compiler/dart2js/mirrors_metadata_test.dart
@@ -3,29 +3,9 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:expect/expect.dart';
-import 'dart:async';
 import "package:async_helper/async_helper.dart";
-import 'dart:io';
-import '../../../sdk/lib/_internal/compiler/implementation/filenames.dart';
-import '../../../sdk/lib/_internal/compiler/implementation/mirrors/mirrors.dart';
-import '../../../sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart';
-import '../../../sdk/lib/_internal/compiler/implementation/source_file_provider.dart';
-import 'mock_compiler.dart';
-
-const String SOURCE = 'source';
-Uri SOURCE_URI = new Uri(scheme: SOURCE, path: SOURCE);
-
-Future<MirrorSystem> createMirrorSystem(String source) {
-  MockCompiler compiler = new MockCompiler(
-      analyzeOnly: true,
-      analyzeAll: true,
-      preserveComments: true);
-    compiler.registerSource(SOURCE_URI, source);
-    compiler.librariesToAnalyzeWhenRun = <Uri>[SOURCE_URI];
-  return compiler.runCompiler(null).then((_) {
-    return new Dart2JsMirrorSystem(compiler);
-  });
-}
+import 'dart:async';
+import 'mirror_system_helper.dart';
 
 void validateDeclarationComment(String code,
                                 String text,