Support async/await in the dart2js frontend.

tests/compiler/dart2js/mock_libraries.dart

 // Copyright (c) 2014, 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 for creating mock versions of platform and internal libraries.
 
 library mock_libraries;
 
 String buildLibrarySource(
     Map<String, String> elementMap,
@@ skipping 24 matching lines @@
       }''',
   'deprecated': 'const Object deprecated = const Deprecated("next release");',
   'double': r'''
       abstract class double extends num {
         static var NAN = 0;
         static parse(s) {}
       }''',
   'Function': 'class Function {}',
   'identical': 'bool identical(Object a, Object b) { return true; }',
   'int': 'abstract class int extends num { }',
+  'Iterable': 'abstract class Iterable {}',
   'LinkedHashMap': r'''
       class LinkedHashMap {
         factory LinkedHashMap._empty() => null;
         factory LinkedHashMap._literal(elements) => null;
       }''',
   'List': r'''
       class List<E> {
         var length;
         List([length]);
         List.filled(length, element);
@@ skipping 295 matching lines @@
 };
 
 const Map<String, String> DEFAULT_ISOLATE_HELPER_LIBRARY =
     const <String, String>{
   'startRootIsolate': 'var startRootIsolate;',
   '_currentIsolate': 'var _currentIsolate;',
   '_callInIsolate': 'var _callInIsolate;',
   '_WorkerBase': 'class _WorkerBase {}',
 };
 
+const Map<String, String> DEFAULT_ASYNC_LIBRARY = const <String, String>{
+  'Future': 'class Future<T> {}',
+  'Stream': 'class Stream<T> {}',
+};
+
 const Map<String, String> DEFAULT_MIRRORS_LIBRARY = const <String, String>{
   'Comment': 'class Comment {}',
   'MirrorSystem': 'class MirrorSystem {}',
   'MirrorsUsed': 'class MirrorsUsed {}',
 };