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

pkg/compiler/lib/src/tree/prettyprint.dart

Index: pkg/compiler/lib/src/tree/prettyprint.dart
diff --git a/pkg/compiler/lib/src/tree/prettyprint.dart b/pkg/compiler/lib/src/tree/prettyprint.dart
index 624848047815851e4defedafad110d86d0c1e86b..f01963b663a7c29a8b6e4828136aa5ce47dfff2a 100644
--- a/pkg/compiler/lib/src/tree/prettyprint.dart
+++ b/pkg/compiler/lib/src/tree/prettyprint.dart
@@ -190,8 +190,12 @@ class PrettyPrinter extends Indentation implements Visitor {
     visitNodeWithChildren(node, "For");
   }
 
-  visitForIn(ForIn node) {
-    openNode(node, "ForIn", {'await': node.awaitToken});
+  visitAsyncForIn(AsyncForIn node) {
+    openNode(node, "AsyncForIn");
+  }
+
+  visitSyncForIn(SyncForIn node) {
+    openNode(node, "ForIn");
     node.visitChildren(this);
     closeNode();
   }