Add 'part of' in dart2js code.

lib/compiler/implementation/dart_backend/utils.dart

Index: lib/compiler/implementation/dart_backend/utils.dart
diff --git a/lib/compiler/implementation/dart_backend/utils.dart b/lib/compiler/implementation/dart_backend/utils.dart
index 20d506d5fed1d314590370a24660d71a1bc422c2..00a40b66f106febb885056af17440d5ee09c3867 100644
--- a/lib/compiler/implementation/dart_backend/utils.dart
+++ b/lib/compiler/implementation/dart_backend/utils.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+part of dart_backend;
+
 class CloningVisitor implements Visitor<Node> {
   final TreeElements originalTreeElements;
   final TreeElementMapping cloneTreeElements;