Add 'part of' in dart2js code.

lib/compiler/implementation/tree/visitors.dart

 // Copyright (c) 2011, 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;
+
 /**
  * This visitor takes another visitor and applies it to every
  * node in the tree. There is currently no way to control the
  * traversal.
  */
 class TraversingVisitor extends Visitor {
   final Visitor visitor;
 
   TraversingVisitor(Visitor this.visitor);
 
   visitNode(Node node) {
     node.accept(visitor);
     node.visitChildren(this);
   }
 }