Remove usage of dart:json.

pkg/http/test/mock_client_test.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.
 
 library mock_client_test;
 
 import 'dart:async';
-import 'dart:json' as json;
+import 'dart:convert';
 
 import 'package:http/http.dart' as http;
 import 'package:http/src/utils.dart';
 import 'package:http/testing.dart';
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 void main() {
   test('handles a request', () {
     var client = new MockClient((request) {
       return new Future.value(new http.Response(
-          json.stringify(request.bodyFields), 200,
+          JSON.encode(request.bodyFields), 200,
           request: request, headers: {'content-type': 'application/json'}));
     });
 
     expect(client.post("http://example.com/foo", fields: {
       'field1': 'value1',
       'field2': 'value2'
     }).then((response) => response.body), completion(parse(equals({
       'field1': 'value1',
       'field2': 'value2'
     }))));
@@ skipping 23 matching lines @@
 
   test('handles a request with no body', () {
     var client = new MockClient((request) {
       return new Future.value(new http.Response('you did it', 200));
     });
 
     expect(client.read("http://example.com/foo"),
         completion(equals('you did it')));
   });
 }