Revert "Parts must start with 'part of'" and "Attempt to fix VM build"

dart/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);
   }
 }