Model 'await for' as an explicit node in tree and fix its handling.

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

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 6d1ead580bb19fe02121db0d90d4273b2ca03c37..94c62a45a1e2c2764232f6a9eef50b5cc54cf5f5 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -2680,6 +2680,11 @@ class JavaScriptResolutionCallbacks extends ResolutionCallbacks {
     registerBackendStaticInvocation(backend.assertMethod, registry);
   }
 
+  void onAsyncForIn(AsyncForIn node, Registry registry) {
+    registerBackendStaticInvocation(backend.getStreamIteratorConstructor(),
+                                    registry);
+  }
+
   void onStringInterpolation(Registry registry) {
     assert(registry.isForResolution);
     registerBackendStaticInvocation(