Add 'part of' in dart2js code.

lib/compiler/implementation/world.dart

Index: lib/compiler/implementation/world.dart
diff --git a/lib/compiler/implementation/world.dart b/lib/compiler/implementation/world.dart
index e79b01606d230fe7df7262358a0d57168338d050..836d47cd8cc5c8ff62dd2e373b9d899f06537bec 100644
--- a/lib/compiler/implementation/world.dart
+++ b/lib/compiler/implementation/world.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 dart2js;
+
 class World {
   final Compiler compiler;
   final Map<ClassElement, Set<ClassElement>> subtypes;