Add 'part of' in dart2js code.

lib/compiler/implementation/tree_validator.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 dart2js;
+
 class TreeValidatorTask extends CompilerTask {
   TreeValidatorTask(Compiler compiler) : super(compiler);
 
   void validate(Node tree) {
     assert(check(tree));
   }
 
   bool check(Node tree) {
     List<InvalidNodeError> errors = [];
     void report(node, message) {
@@ skipping 52 matching lines @@
   final String message;
   InvalidNodeError(this.node, [this.message]);
 
   toString() {
     String nodeString = node.toDebugString();
     String result = 'invalid node: $nodeString';
     if (message != null) result = '$result ($message)';
     return result;
   }
 }