Add 'part of' in dart2js code.

lib/compiler/implementation/universe/selector_map.dart

Index: lib/compiler/implementation/universe/selector_map.dart
diff --git a/lib/compiler/implementation/universe/selector_map.dart b/lib/compiler/implementation/universe/selector_map.dart
index b1861ebcbc71b9de53744407d47427c31693cb5a..f4a416d5d00a27a2b5215d08899b055f124d922c 100644
--- a/lib/compiler/implementation/universe/selector_map.dart
+++ b/lib/compiler/implementation/universe/selector_map.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;
+
 class SelectorMap<T> extends PartialTypeTree {
 
   SelectorMap(Compiler compiler) : super(compiler);