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

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

 // Copyright (c) 2012, 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 js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 const bool USE_CPS_IR = const bool.fromEnvironment("USE_CPS_IR");
 
@@ skipping 2662 matching lines @@
   void registerBackendInstantiation(ClassElement element, Registry registry) {
     backend.registerBackendUse(element);
     element.ensureResolved(backend.compiler);
     registry.registerInstantiation(element.rawType);
   }
 
   void onAssert(Send node, Registry registry) {
     registerBackendStaticInvocation(backend.assertMethod, registry);
   }
 
+  void onAsyncForIn(AsyncForIn node, Registry registry) {
+    registerBackendStaticInvocation(backend.getStreamIteratorConstructor(),
+                                    registry);
+  }
+
   void onStringInterpolation(Registry registry) {
     assert(registry.isForResolution);
     registerBackendStaticInvocation(
         backend.getStringInterpolationHelper(), registry);
   }
 
   void onCatchStatement(Registry registry) {
     assert(registry.isForResolution);
     registerBackendStaticInvocation(backend.getExceptionUnwrapper(), registry);
     registerBackendInstantiation(
@@ skipping 171 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }