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

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

Index: pkg/compiler/lib/src/js_backend/backend_helpers.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend_helpers.dart b/pkg/compiler/lib/src/js_backend/backend_helpers.dart
index 2650a7e940ee73f8125ba8af90f832b34ed66c79..affed935385c6c6a7057d40f4ffe737770ea9541 100644
--- a/pkg/compiler/lib/src/js_backend/backend_helpers.dart
+++ b/pkg/compiler/lib/src/js_backend/backend_helpers.dart
@@ -586,6 +586,18 @@ class BackendHelpers {
     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);
@@ -667,4 +679,4 @@ class BackendHelpers {
   Element get convertRtiToRuntimeType {
     return findHelper('convertRtiToRuntimeType');
   }
-}
+}