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

pkg/compiler/lib/src/typechecker.dart

Index: pkg/compiler/lib/src/typechecker.dart
diff --git a/pkg/compiler/lib/src/typechecker.dart b/pkg/compiler/lib/src/typechecker.dart
index 04a7a432fff6f91592c8d1c2062b1670e7b292dc..078086c823def58beca12c5ff43afed21d522686 100644
--- a/pkg/compiler/lib/src/typechecker.dart
+++ b/pkg/compiler/lib/src/typechecker.dart
@@ -1719,7 +1719,13 @@ class TypeCheckerVisitor extends Visitor<DartType> {
     return const StatementType();
   }
 
-  visitForIn(ForIn node) {
+  visitAsyncForIn(AsyncForIn node) {
+    analyze(node.expression);
+    analyze(node.body);
+    return const StatementType();
+  }
+
+  visitSyncForIn(SyncForIn node) {
     analyze(node.expression);
     analyze(node.body);
     return const StatementType();