Rename and fix json frog library in SDK

frog/lib/json_frog.js

 // Copyright (c) 2011, 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.
 
 // JavaScript implementation of the native methods declared in json.dart.
 
 // TODO(jmesserly): this needs lots of cleanup.
 // Ideally JS objects could be treated as Dart Maps directly, and then we can
 // really use native JSON.parse and JSON.stringify.
 
@@ skipping 66 matching lines @@
 // Converts only shallowly.
 function $convertDartMapToJsObject(map) {
   var obj = {};
   var propertyNames = map.getKeys();
   for (var i = 0, len = propertyNames.length; i < len; i++) {
     var propertyName = propertyNames[i];
     obj[propertyName] = map.$index(propertyName);
   }
   return obj;
 }