Add 'part of' in dart2js code.

lib/compiler/implementation/tree/dartstring.dart

Index: lib/compiler/implementation/tree/dartstring.dart
diff --git a/lib/compiler/implementation/tree/dartstring.dart b/lib/compiler/implementation/tree/dartstring.dart
index 9470ded345752294ef8b75edb0cad56f38b269f3..ac50e270b670718651b7a58f95999e4a2f3edb3e 100644
--- a/lib/compiler/implementation/tree/dartstring.dart
+++ b/lib/compiler/implementation/tree/dartstring.dart
@@ -2,6 +2,8 @@
 // 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.