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

pkg/compiler/lib/src/common/backend_api.dart

Index: pkg/compiler/lib/src/common/backend_api.dart
diff --git a/pkg/compiler/lib/src/common/backend_api.dart b/pkg/compiler/lib/src/common/backend_api.dart
index bdfcf79d0edd66ad46c7ed48ec8c17d0e0e2344f..cb52c7f3894ce28cb3302d8f4537d8d07bb72adc 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -250,6 +250,12 @@ abstract class Backend {
   ClassElement get uint32Implementation => compiler.coreClasses.intClass;
   ClassElement get uint31Implementation => compiler.coreClasses.intClass;
   ClassElement get positiveIntImplementation => compiler.coreClasses.intClass;
+  ClassElement get syncStarIterableImplementation =>
+      compiler.coreClasses.iterableClass;
+  ClassElement get asyncFutureImplementation =>
+      compiler.coreClasses.futureClass;
+  ClassElement get asyncStarStreamImplementation =>
+      compiler.coreClasses.streamClass;
 
   ClassElement defaultSuperclass(ClassElement element) {
     return compiler.coreClasses.objectClass;