Add 'part of' in dart2js code.

lib/compiler/implementation/js_backend/constant_emitter.dart

Index: lib/compiler/implementation/js_backend/constant_emitter.dart
diff --git a/lib/compiler/implementation/js_backend/constant_emitter.dart b/lib/compiler/implementation/js_backend/constant_emitter.dart
index af520c3cd9d1a5207e2929c3391b302eb8765b01..ad7b2b055f31b680a7b176a195af55da1fb5f454 100644
--- a/lib/compiler/implementation/js_backend/constant_emitter.dart
+++ b/lib/compiler/implementation/js_backend/constant_emitter.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 js_backend;
+
 class ConstantEmitter implements ConstantVisitor {
   final Compiler compiler;
   final Namer namer;