Remove usage of dart:json.

sdk/lib/_internal/pub/test/oauth2/with_an_expired_credentials_refreshes_and_saves_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';
 
 main() {
   initConfig();
@@ skipping 11 matching lines @@
     confirmPublish(pub);
 
     server.handle('POST', '/token', (request) {
       return new ByteStream(request).toBytes().then((bytes) {
         var body = new String.fromCharCodes(bytes);
         expect(body, matches(
             new RegExp(r'(^|&)refresh_token=refresh\+token(&|$)')));
 
         request.response.headers.contentType =
             new ContentType("application", "json");
-        request.response.write(json.stringify({
+        request.response.write(JSON.encode({
           "access_token": "new access token",
           "token_type": "bearer"
         }));
         request.response.close();
       });
     });
 
     server.handle('GET', '/api/packages/versions/new', (request) {
       expect(request.headers.value('authorization'),
           equals('Bearer new access token'));
 
       request.response.close();
     });
 
     pub.shouldExit();
 
     d.credentialsFile(server, 'new access token', refreshToken: 'refresh token')
         .validate();
   });
 }