Add 'part of' in dart2js code.

lib/compiler/implementation/dart_backend/renamer.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // 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;
+
 Function get _compareNodes =>
     compareBy((n) => n.getBeginToken().charOffset);
 
 typedef String _Renamer(Renamable renamable);
 abstract class Renamable {
   const int RENAMABLE_TYPE_ELEMENT = 1;
   const int RENAMABLE_TYPE_MEMBER = 2;
   const int RENAMABLE_TYPE_LOCAL = 3;
 
   final Set<Node> nodes;
@@ skipping 270 matching lines @@
     index ~/= firstCharAlphabet.length;
     int length = otherCharsAlphabet.length;
     while (index >= length) {
       resultBuilder.add(otherCharsAlphabet[index % length]);
       index ~/= length;
     }
     resultBuilder.add(otherCharsAlphabet[index]);
     return resultBuilder.toString();
   }
 }