Add 'part of' in dart2js code.

lib/compiler/implementation/dart_backend/emitter.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 dart_backend;
+
 String emitCode(
       Unparser unparser,
       Map<LibraryElement, String> imports,
       Collection<Node> topLevelNodes,
       Map<ClassNode, Collection<Node>> classMembers) {
   imports.forEach((libraryElement, prefix) {
     unparser.unparseImportTag('${libraryElement.uri}', prefix);
   });
 
   for (final node in topLevelNodes) {
     if (node is ClassNode) {
       // TODO(smok): Filter out default constructors here.
       unparser.unparseClassWithBody(node, classMembers[node]);
     } else {
       unparser.unparse(node);
     }
   }
 }