Add 'part of' in dart2js code.

lib/compiler/implementation/js_backend/native_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 js_backend;
+
 class NativeEmitter {
 
   CodeEmitterTask emitter;
   CodeBuffer nativeBuffer;
 
   // Classes that participate in dynamic dispatch. These are the
   // classes that contain used members.
   Set<ClassElement> classesWithDynamicDispatch;
 
   // Native classes found in the application.
@@ skipping 458 matching lines @@
         if (!first) targetBuffer.add(",\n");
         targetBuffer.add(" $name: $function");
         first = false;
       });
       targetBuffer.add("\n});\n\n");
     }
     targetBuffer.add(nativeBuffer);
     targetBuffer.add('\n');
   }
 }