Add 'part of' in dart2js code.

lib/compiler/implementation/diagnostic_listener.dart

Index: lib/compiler/implementation/diagnostic_listener.dart
diff --git a/lib/compiler/implementation/diagnostic_listener.dart b/lib/compiler/implementation/diagnostic_listener.dart
index e684e9eb7e3a4e843e7b1aaf8017bd60daf4d67f..fa335e0e5f4522d70554a029413bc439916a6676 100644
--- a/lib/compiler/implementation/diagnostic_listener.dart
+++ b/lib/compiler/implementation/diagnostic_listener.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 dart2js;
+
 abstract class DiagnosticListener {
   // TODO(karlklose): replace cancel with better error reporting mechanism.
   void cancel(String reason, {node, token, instruction, element});