js_runtime: Split _asyncHelper into separate functions

pkg/compiler/lib/src/js_backend/backend_impact.dart

 // Copyright (c) 2015, 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 dart2js.js_helpers.impact;
 
 import '../common/names.dart';
 import '../common_elements.dart' show CommonElements, ElementEnvironment;
 import '../elements/types.dart' show InterfaceType;
 import '../elements/entities.dart';
@@ skipping 111 matching lines @@
     return _mainWithArguments ??= new BackendImpact(instantiatedClasses: [
       _commonElements.jsArrayClass,
       _commonElements.jsStringClass
     ]);
   }
 
   BackendImpact _asyncBody;
 
   BackendImpact get asyncBody {
     return _asyncBody ??= new BackendImpact(staticUses: [
-      _commonElements.asyncHelper,
+      _commonElements.asyncHelperStart,
+      _commonElements.asyncHelperAwait,
+      _commonElements.asyncHelperReturn,
+      _commonElements.asyncHelperRethrow,
       _commonElements.syncCompleterConstructor,
       _commonElements.streamIteratorConstructor,
       _commonElements.wrapBody
     ]);
   }
 
   BackendImpact _syncStarBody;
 
   BackendImpact get syncStarBody {
     return _syncStarBody ??= new BackendImpact(staticUses: [
@@ skipping 646 matching lines @@
   /// Backend impact for type variables through mirrors.
   BackendImpact get typeVariableMirror {
     return _typeVariableMirror ??= new BackendImpact(staticUses: [
       _commonElements.typeVariableConstructor,
       _commonElements.createRuntimeType
     ], instantiatedClasses: [
       _commonElements.typeVariableClass
     ]);
   }
 }