Add 'part of' in dart2js code.

lib/compiler/implementation/universe/partial_type_tree.dart

Index: lib/compiler/implementation/universe/partial_type_tree.dart
diff --git a/lib/compiler/implementation/universe/partial_type_tree.dart b/lib/compiler/implementation/universe/partial_type_tree.dart
index ae6e9b961ed49ef9984582b5d78e19b64df6ab15..cbe1be0dbb845bde0428406ba28d44fcd147acdb 100644
--- a/lib/compiler/implementation/universe/partial_type_tree.dart
+++ b/lib/compiler/implementation/universe/partial_type_tree.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 universe;
+
 abstract class PartialTypeTree {
 
   final Compiler compiler;