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

pkg/compiler/lib/src/scanner/listener.dart

Index: pkg/compiler/lib/src/scanner/listener.dart
diff --git a/pkg/compiler/lib/src/scanner/listener.dart b/pkg/compiler/lib/src/scanner/listener.dart
index 0ef1f5db15af6991c96040f33180819a647c0260..568577a86c183b060d20921c58f35bfbf7fa7d4a 100644
--- a/pkg/compiler/lib/src/scanner/listener.dart
+++ b/pkg/compiler/lib/src/scanner/listener.dart
@@ -2161,8 +2161,13 @@ class NodeListener extends ElementListener {
     Statement body = popNode();
     Expression expression = popNode();
     Node declaredIdentifier = popNode();
-    pushNode(new ForIn(declaredIdentifier, expression, body,
-                       awaitToken, forToken, inKeyword));
+    if (awaitToken == null) {
+      pushNode(new SyncForIn(declaredIdentifier, expression, body,
+                             forToken, inKeyword));
+    } else {
+      pushNode(new AsyncForIn(declaredIdentifier, expression, body, awaitToken,
+                              forToken, inKeyword));
+    }
   }
 
   void endMetadataStar(int count, bool forParameter) {