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

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 218 matching lines @@
   'TypeImpl': 'class TypeImpl {}',
   'TypeVariable': 'class TypeVariable {}',
   'unwrapException': 'unwrapException(e) {}',
   'voidTypeCheck': 'voidTypeCheck(value) {}',
   'wrapException': 'wrapException(x) { return x; }',
   'badMain': 'badMain() { throw "bad main"; }',
   'missingMain': 'missingMain() { throw "missing main"; }',
   'mainHasTooManyParameters':
       'mainHasTooManyParameters() '
       '{ throw "main has too many parameters"; }',
-  '_wrapJsFunctionForAsync': '_wrapJsFunctionForAsync(f) {}',
 };
 
 const Map<String, String> DEFAULT_FOREIGN_HELPER_LIBRARY
     = const <String, String>{
   'JS': r'''
       dynamic JS(String typeDescription, String codeTemplate,
         [var arg0, var arg1, var arg2, var arg3, var arg4, var arg5, var arg6,
          var arg7, var arg8, var arg9, var arg10, var arg11]) {}''',
 };
 
@@ skipping 130 matching lines @@
       '''
       class Future<T> {
         Future.value([value]);
       }
       ''',
   'Stream': 'class Stream<T> {}',
   'Completer': 'class Completer<T> {}',
   'StreamIterator': 'class StreamIterator<T> {}',
 };
 
+/// These members are only needed when async/await is used.
+const Map<String, String> ASYNC_AWAIT_LIBRARY = const <String, String>{
+  '_wrapJsFunctionForAsync': '_wrapJsFunctionForAsync(f) {}',
+  '_asyncHelper': '_asyncHelper(o, f, c) {}',
+};
+
 const Map<String, String> DEFAULT_MIRRORS_LIBRARY = const <String, String>{
   'Comment': 'class Comment {}',
   'MirrorSystem': 'class MirrorSystem {}',
   'MirrorsUsed': 'class MirrorsUsed {}',
 };