dart2js: Don't zone-register callbacks in async functions for every await.

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 99 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));
+
+    Map<String, String> asyncLibrarySource = <String, String>{};
+    asyncLibrarySource.addAll(DEFAULT_ASYNC_LIBRARY);
+    if (enableAsyncAwait) {
+      asyncLibrarySource.addAll(ASYNC_AWAIT_LIBRARY);
+    }
     registerSource(Compiler.DART_ASYNC,
-                   buildLibrarySource(DEFAULT_ASYNC_LIBRARY));
+                   buildLibrarySource(asyncLibrarySource));
   }
 
   String get patchVersion {
     return testedPatchVersion != null ? testedPatchVersion : super.patchVersion;
   }
 
   /// Initialize the mock compiler with an empty main library.
   Future<Uri> init([String mainSource = ""]) {
     Uri uri = new Uri(scheme: "mock");
     registerSource(uri, mainSource);
@@ skipping 272 matching lines @@
   MockElement(Element enclosingElement)
       : super('', ElementKind.FUNCTION, Modifiers.EMPTY,
               enclosingElement);
 
   get node => null;
 
   parseNode(_) => null;
 
   bool get hasNode => false;
 }