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