Add 'part of' in dart2js code.

lib/compiler/implementation/world.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // 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;
   final Set<ClassElement> classesNeedingRti;
   final Map<ClassElement, Set<ClassElement>> rtiDependencies;
   final FunctionSet userDefinedGetters;
   final FunctionSet userDefinedSetters;
 
   World(Compiler compiler)
       : subtypes = new Map<ClassElement, Set<ClassElement>>(),
@@ skipping 169 matching lines @@
   final SourceString name;
 
   MemberSet(SourceString this.name) : elements = new Set<Element>();
 
   void add(Element element) {
     elements.add(element);
   }
 
   bool get isEmpty => elements.isEmpty;
 }