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

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

 // Copyright (c) 2014, 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 resolution;
 
 /// [ResolutionRegistry] collects all resolution information. It stores node
 /// related information in a [TreeElements] mapping and registers calls with
 /// [Backend], [World] and [Enqueuer].
 // TODO(johnniwinther): Split this into an interface and implementation class.
@@ skipping 354 matching lines @@
     backend.resolutionCallbacks.onAssert(node, this);
   }
 
   bool isAssert(Send node) {
     return mapping.isAssert(node);
   }
 
   void registerAsyncMarker(FunctionElement element) {
     backend.registerAsyncMarker(element, world, this);
   }
+
+  void registerAsyncForIn(AsyncForIn node) {
+    backend.resolutionCallbacks.onAsyncForIn(node, this);
+  }
 }