Remove usage of dart:json.

sdk/lib/_internal/pub/test/oauth2/utils.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 oauth2.utils;
 
+import 'dart:convert';
 import 'dart:io';
-import 'dart:json' as json;
 
 import 'package:http/http.dart' as http;
 import 'package:scheduled_test/scheduled_process.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:scheduled_test/scheduled_server.dart';
 
 import '../../lib/src/io.dart';
 import '../../lib/src/utils.dart';
 
 void authorizePub(ScheduledProcess pub, ScheduledServer server,
@@ skipping 19 matching lines @@
 }
 
 void handleAccessTokenRequest(ScheduledServer server, String accessToken) {
   server.handle('POST', '/token', (request) {
     return new ByteStream(request).toBytes().then((bytes) {
       var body = new String.fromCharCodes(bytes);
       expect(body, matches(new RegExp(r'(^|&)code=access\+code(&|$)')));
 
       request.response.headers.contentType =
           new ContentType("application", "json");
-      request.response.write(json.stringify({
+      request.response.write(JSON.encode({
         "access_token": accessToken,
         "token_type": "bearer"
       }));
       request.response.close();
     });
   });
 }