Add 'part of' in dart2js code.

lib/compiler/implementation/js_backend/constant_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 ConstantEmitter implements ConstantVisitor {
   final Compiler compiler;
   final Namer namer;
 
   CodeBuffer buffer;
   bool shouldEmitCanonicalVersion;
 
   ConstantEmitter(this.compiler, this.namer);
 
   /**
@@ skipping 228 matching lines @@
       buffer.add(getJsConstructor(constant.type.element));
       buffer.add("(");
       for (int i = 0; i < constant.fields.length; i++) {
         if (i != 0) buffer.add(", ");
         _visit(constant.fields[i]);
       }
       buffer.add(")");
     }
   }
 }