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

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 5bd9aeede472827a81b4c4bdc8de544f97cde059..6bbcd29a28ca97fef1c43155b35014a77cc2a64a 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -530,6 +530,9 @@ class ResolutionCallbacks {
   /// Register that [node] is a call to `assert`.
   void onAssert(Send node, Registry registry) {}
 
+  /// Register that an 'await for' has been seen.
+  void onAsyncForIn(AsyncForIn node, Registry registry) {}
+
   /// Called during resolution to notify to the backend that the
   /// program uses string interpolation.
   void onStringInterpolation(Registry registry) {}