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

tests/compiler/dart2js/mock_libraries.dart

Index: tests/compiler/dart2js/mock_libraries.dart
diff --git a/tests/compiler/dart2js/mock_libraries.dart b/tests/compiler/dart2js/mock_libraries.dart
index 6db960c45375c7d2f042d6a1583d754555490dd0..b0bcaa16112d13769541ebdb4a3f44942274735b 100644
--- a/tests/compiler/dart2js/mock_libraries.dart
+++ b/tests/compiler/dart2js/mock_libraries.dart
@@ -236,7 +236,6 @@ const Map<String, String> DEFAULT_JS_HELPER_LIBRARY = const <String, String>{
   'mainHasTooManyParameters':
       'mainHasTooManyParameters() '
       '{ throw "main has too many parameters"; }',
-  '_wrapJsFunctionForAsync': '_wrapJsFunctionForAsync(f) {}',
 };
 
 const Map<String, String> DEFAULT_FOREIGN_HELPER_LIBRARY
@@ -387,6 +386,12 @@ const Map<String, String> DEFAULT_ASYNC_LIBRARY = const <String, String>{
   '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 {}',