Remove usage of dart:json.

sdk/lib/_internal/pub/test/oauth2/with_a_server_rejected_refresh_token_authenticates_again_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.
 
+import 'dart:convert';
 import 'dart:io';
-import 'dart:json' as json;
 
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:scheduled_test/scheduled_server.dart';
 
 import '../../lib/src/io.dart';
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
 main() {
@@ skipping 12 matching lines @@
     var pub = startPublish(server);
 
     confirmPublish(pub);
 
     server.handle('POST', '/token', (request) {
       return new ByteStream(request).toBytes().then((bytes) {
         var response = request.response;
         response.statusCode = 400;
         response.reasonPhrase = 'Bad request';
         response.headers.contentType = new ContentType("application", "json");
-        response.write(json.stringify({"error": "invalid_request"}));
+        response.write(JSON.encode({"error": "invalid_request"}));
         response.close();
       });
     });
 
     expect(pub.nextLine(), completion(matches(r'Uploading\.\.\.+')));
     authorizePub(pub, server, 'new access token');
 
     server.handle('GET', '/api/packages/versions/new', (request) {
       expect(request.headers.value('authorization'),
           equals('Bearer new access token'));
 
       request.response.close();
     });
 
     pub.kill();
   });
 }