Move json to lib/json/{json.dart, json_frog.dart}

lib/json/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');
 #import('dart:coreimpl');
 
 // TODO(jmesserly): this needs cleanup
 // Ideally JS objects could be treated as Dart Maps directly, and then we can
 // really use native JSON.parse and JSON.stringify.
@@ skipping 69 matching lines @@
       if (obj is Map<String, Dynamic>) {
         Map<String, Dynamic> map = obj;
         obj = new Object();
         map.forEach((k, v) => _setValue(obj, k, v));
         return obj;
       }
       throw new IllegalArgumentException('cannot convert "$value" to JSON');
     });
   }
 }