Add 'part of' in dart2js code.

lib/compiler/implementation/tree/prettyprint.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 tree;
+
 /**
  * Pretty-prints Node tree in XML-like format.
  *
  * TODO(smok): Add main() to run from command-line to print out tree for given
  * .dart file.
  */
 class PrettyPrinter implements Visitor {
 
   /** String used to represent one level of indent. */
   static const String INDENT = "  ";
@@ skipping 446 matching lines @@
   }
 
   visitStringNode(StringNode node) {
     unimplemented('visitNode', node: node);
   }
 
   unimplemented(String message, {Node node}) {
     throw message;
   }
 }