Rename and fix json frog library in SDK

frog/lib/json.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");
-
-/**
- * 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:
-   *   null
-   *   a bool
-   *   a double
-   *   a String
-   *   an Array of values (recursively)
-   *   a Map from property names to values (recursively)
-   */
-  static Object parse(String jsonString) native;
-
-  /**
-   * Takes a value and returns a string in JSON notation
-   * representing its value, or returns null if the value is not representable
-   * 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;
-}