Support async/await in the dart2js frontend.

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2012, 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 mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:collection';
 
@@ skipping 89 matching lines @@
     registerSource(JavaScriptBackend.DART_JS_HELPER,
                    buildLibrarySource(DEFAULT_JS_HELPER_LIBRARY));
     registerSource(JavaScriptBackend.DART_FOREIGN_HELPER,
                    buildLibrarySource(DEFAULT_FOREIGN_HELPER_LIBRARY));
     registerSource(JavaScriptBackend.DART_INTERCEPTORS,
                    buildLibrarySource(DEFAULT_INTERCEPTORS_LIBRARY));
     registerSource(JavaScriptBackend.DART_ISOLATE_HELPER,
                    buildLibrarySource(DEFAULT_ISOLATE_HELPER_LIBRARY));
     registerSource(Compiler.DART_MIRRORS,
                    buildLibrarySource(DEFAULT_MIRRORS_LIBRARY));
+    registerSource(Compiler.DART_ASYNC,
+                   buildLibrarySource(DEFAULT_ASYNC_LIBRARY));
   }
 
   /// Initialize the mock compiler with an empty main library.
   Future<Uri> init([String mainSource = ""]) {
     Uri uri = new Uri(scheme: "mock");
     registerSource(uri, mainSource);
     return libraryLoader.loadLibrary(uri)
         .then((LibraryElement library) {
       mainApp = library;
       // We need to make sure the Object class is resolved. When registering a
@@ skipping 267 matching lines @@
   MockElement(Element enclosingElement)
       : super('', ElementKind.FUNCTION, Modifiers.EMPTY,
               enclosingElement, false);
 
   get node => null;
 
   parseNode(_) => null;
 
   bool get hasNode => false;
 }