Remove usage of dart:json.

pkg/http/lib/testing.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.
 
 /// This library contains testing classes for the HTTP library.
 ///
 /// The [MockClient] class is a drop-in replacement for `http.Client` that
 /// allows test code to set up a local request handler in order to fake a server
 /// that responds to HTTP requests:
 ///
-///     import 'dart:json' as json;
+///     import 'dart:convert';
 ///     import 'package:http/testing.dart';
 ///
 ///     var client = new MockClient((request) {
 ///       if (request.url.path != "/data.json") {
 ///         return new Response("", 404);
 ///       }
-///       return new Response(json.stringify({
+///       return new Response(JSON.encode({
 ///         'numbers': [1, 4, 15, 19, 214]
 ///       }, 200, headers: {
 ///         'content-type': 'application/json'
 ///       });
 ///     };
 library http.testing;
 
 export 'src/mock_client.dart';