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

pkg/compiler/lib/src/resolution/registry.dart

Index: pkg/compiler/lib/src/resolution/registry.dart
diff --git a/pkg/compiler/lib/src/resolution/registry.dart b/pkg/compiler/lib/src/resolution/registry.dart
index 15082856f66d10c8c35825f2870065cdcd6b0de0..8b37559d0bffb36df915917b2bc93cf0e3958d72 100644
--- a/pkg/compiler/lib/src/resolution/registry.dart
+++ b/pkg/compiler/lib/src/resolution/registry.dart
@@ -372,4 +372,8 @@ class ResolutionRegistry extends Registry {
   void registerAsyncMarker(FunctionElement element) {
     backend.registerAsyncMarker(element, world, this);
   }
+
+  void registerAsyncForIn(AsyncForIn node) {
+    backend.resolutionCallbacks.onAsyncForIn(node, this);
+  }
 }