Add 'part of' in dart2js code.

lib/compiler/implementation/code_buffer.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 dart2js;
+
 class CodeBuffer implements StringBuffer {
   StringBuffer buffer;
   List<CodeBufferMarker> markers;
   int lastBufferOffset = 0;
   int mappedRangeCounter = 0;
 
   CodeBuffer()
       : buffer = new StringBuffer(),
         markers = new List<CodeBufferMarker>();
 
@@ skipping 76 matching lines @@
     });
   }
 }
 
 class CodeBufferMarker {
   final int offsetDelta;
   final sourcePosition;
 
   CodeBufferMarker(this.offsetDelta, this.sourcePosition);
 }