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

pkg/compiler/lib/src/js_backend/backend.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.
 
 part of js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 1860 matching lines @@
   ClassElement get stringImplementation => helpers.jsStringClass;
   ClassElement get listImplementation => helpers.jsArrayClass;
   ClassElement get constListImplementation => helpers.jsUnmodifiableArrayClass;
   ClassElement get fixedListImplementation => helpers.jsFixedArrayClass;
   ClassElement get growableListImplementation => helpers.jsExtendableArrayClass;
   ClassElement get mapImplementation => helpers.mapLiteralClass;
   ClassElement get constMapImplementation => helpers.constMapLiteralClass;
   ClassElement get typeImplementation => helpers.typeLiteralClass;
   ClassElement get boolImplementation => helpers.jsBoolClass;
   ClassElement get nullImplementation => helpers.jsNullClass;
+  ClassElement get syncStarIterableImplementation => helpers.syncStarIterable;
+  ClassElement get asyncFutureImplementation => helpers.futureImplementation;
+  ClassElement get asyncStarStreamImplementation => helpers.controllerStream;
 
   void registerStaticUse(Element element, Enqueuer enqueuer) {
     if (element == helpers.disableTreeShakingMarker) {
       compiler.disableTypeInferenceForMirrors = true;
       isTreeShakingDisabled = true;
     } else if (element == helpers.preserveNamesMarker) {
       mustPreserveNames = true;
     } else if (element == helpers.preserveMetadataMarker) {
       mustRetainMetadata = true;
     } else if (element == helpers.preserveUrisMarker) {
@@ skipping 1110 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }