Remove usage of dart:json.

pkg/http/test/mock_client_test.dart

Index: pkg/http/test/mock_client_test.dart
diff --git a/pkg/http/test/mock_client_test.dart b/pkg/http/test/mock_client_test.dart
index 997f0bb9c14447270a3e2e8f14c20c1647cbd903..07066ec8f27ff608a466fa05df26b4cf94da529d 100644
--- a/pkg/http/test/mock_client_test.dart
+++ b/pkg/http/test/mock_client_test.dart
@@ -5,7 +5,7 @@
 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';
@@ -18,7 +18,7 @@ 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'}));
     });