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

dart/lib/compiler/implementation/tree/visitors.dart

Index: dart/lib/compiler/implementation/tree/visitors.dart
diff --git a/dart/lib/compiler/implementation/tree/visitors.dart b/dart/lib/compiler/implementation/tree/visitors.dart
index 9096cff5b08217965263b40966aaa04ab7b4d3a2..bab85b0b5e59a0e0b902c84f8ef740f7d6f61c79 100644
--- a/dart/lib/compiler/implementation/tree/visitors.dart
+++ b/dart/lib/compiler/implementation/tree/visitors.dart
@@ -2,8 +2,6 @@
 // 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