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

dart/lib/compiler/implementation/tree/dartstring.dart

Index: dart/lib/compiler/implementation/tree/dartstring.dart
diff --git a/dart/lib/compiler/implementation/tree/dartstring.dart b/dart/lib/compiler/implementation/tree/dartstring.dart
index 4720c57e66dd054e360dfbd7790795109eea90b7..0d7f70c3077b42b39976066729d05457f88c754d 100644
--- a/dart/lib/compiler/implementation/tree/dartstring.dart
+++ b/dart/lib/compiler/implementation/tree/dartstring.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;
-
 /**
  * The [DartString] type represents a Dart string value as a sequence of Unicode
  * Scalar Values.