dart2js: Set inferred return types for async, async*, and sync* methods.

pkg/compiler/lib/src/js_backend/backend_helpers.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_backend.helpers;
 
 import '../common.dart';
 import '../common/names.dart' show
     Uris;
 import '../common/resolution.dart' show
@@ skipping 568 matching lines @@
     classElement.ensureResolved(resolution);
     return classElement.lookupLocalMember("endOfIteration");
   }
 
   Element get syncStarIterable {
     ClassElement classElement = findAsyncHelper("_SyncStarIterable");
     classElement.ensureResolved(resolution);
     return classElement;
   }
 
+  Element get futureImplementation {
+    ClassElement classElement = findAsyncHelper('_Future');
+    classElement.ensureResolved(resolution);
+    return classElement;
+  }
+
+  Element get controllerStream {
+    ClassElement classElement = findAsyncHelper("_ControllerStream");
+    classElement.ensureResolved(resolution);
+    return classElement;
+  }
+
   Element get syncStarIterableConstructor {
     ClassElement classElement = syncStarIterable;
     classElement.ensureResolved(resolution);
     return classElement.lookupConstructor("");
   }
 
   Element get syncCompleterConstructor {
     ClassElement classElement = find(compiler.asyncLibrary, "Completer");
     classElement.ensureResolved(resolution);
     return classElement.lookupConstructor("sync");
@@ skipping 61 matching lines @@
     return find(isolateHelperLibrary, '_callInIsolate');
   }
 
   Element get findIndexForNativeSubclassType {
     return findInterceptor('findIndexForNativeSubclassType');
   }
 
   Element get convertRtiToRuntimeType {
     return findHelper('convertRtiToRuntimeType');
   }
-}
+}