Rename and fix json frog library in SDK

frog/lib/json_frog.dart

 // 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.
 
 #library("json");
-#native("json.js");
+#native("json_frog.js");
 
 /**
  * Dart interface to JavaScript objects and JSON.
  * (The native method implementations are in json.js.)
  */
 class JSON native 'JSON' {
 
   /**
    * Takes a string in JSON notation and returns the value it
    * represents.  The resulting value is one of the following:
@@ skipping 12 matching lines @@
    * in JSON.  A representable value is one of the following:
    *   null
    *   a bool
    *   a double
    *   a String
    *   an Array of values (recursively)
    *   a Map from property names to values (recursively)
    */
   static String stringify(Object value) native;
 }