Make dart2js unittests async.

tests/compiler/dart2js/size_test.dart

 // Copyright (c) 2013, 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 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import "compiler_helper.dart";
 
 const String TEST = "main() => [];";
 
@@ skipping 11 matching lines @@
   class Type {}
   class Map {}
   class StackTrace {}
   identical(a, b) => true;
   const proxy = 0;
 ''';
 
 main() {
   asyncTest(() => compileAll(TEST, coreSource: DEFAULT_CORELIB_WITH_LIST).
       then((generated) {
-    MockCompiler compiler = new MockCompiler();
-    var backend = compiler.backend;
+    return MockCompiler.create((MockCompiler compiler) {
+      var backend = compiler.backend;
 
-    // Make sure no class is emitted.
-    Expect.isFalse(generated.contains(backend.emitter.finishClassesName));
+      // Make sure no class is emitted.
+      Expect.isFalse(generated.contains(backend.emitter.finishClassesName));
+    });
   }));
 }