Remove usage of dart:json.

sdk/lib/_internal/pub/test/lish/upload_form_fields_is_not_a_map_test.dart

 // Copyright (c) 2013, 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:json' as json;
+import 'dart:convert';
 
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:scheduled_test/scheduled_server.dart';
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
 main() {
   initConfig();
   setUp(d.validPackage.create);
 
   integration('upload form fields is not a map', () {
     var server = new ScheduledServer();
     d.credentialsFile(server, 'access token').create();
     var pub = startPublish(server);
 
     confirmPublish(pub);
 
     var body = {'url': 'http://example.com/upload', 'fields': 12};
     handleUploadForm(server, body);
     expect(pub.nextErrLine(), completion(equals('Invalid server response:')));
-    expect(pub.nextErrLine(), completion(equals(json.stringify(body))));
+    expect(pub.nextErrLine(), completion(equals(JSON.encode(body))));
     pub.shouldExit(1);
   });
 }