Remove usage of dart:json.

sdk/lib/_internal/pub/test/oauth2/with_server_rejected_credentials_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 '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 main() {
   initConfig();
   integration('with server-rejected credentials, authenticates again and saves '
       'credentials.json', () {
     d.validPackage.create();
     var server = new ScheduledServer();
     d.credentialsFile(server, 'access token').create();
     var pub = startPublish(server);
 
     confirmPublish(pub);
 
     server.handle('GET', '/api/packages/versions/new', (request) {
       var response = request.response;
       response.statusCode = 401;
       response.headers.set('www-authenticate', 'Bearer error="invalid_token",'
           ' error_description="your token sucks"');
-      response.write(json.stringify({
+      response.write(JSON.encode({
         'error': {'message': 'your token sucks'}
       }));
       response.close();
     });
 
     expect(pub.nextErrLine(), completion(equals('OAuth2 authorization failed '
         '(your token sucks).')));
     // TODO(rnystrom): The confirm line is run together with this one because
     // in normal usage, the user will have entered a newline on stdin which
     // gets echoed to the terminal. Do something better here?
     expect(pub.nextLine(), completion(equals('Uploading...')));
     pub.kill();
   });
 }