Remove usage of dart:json.

sdk/lib/_internal/pub/test/lish/package_validation_has_a_warning_and_continues_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('package validation has a warning and continues', () {
     var pkg = packageMap("test_pkg", "1.0.0");
     pkg["author"] = "Nathan Weizenbaum";
     d.dir(appPath, [d.pubspec(pkg)]).create();
 
     var server = new ScheduledServer();
     d.credentialsFile(server, 'access token').create();
     var pub = startPublish(server);
     pub.writeLine("y");
     handleUploadForm(server);
     handleUpload(server);
 
     server.handle('GET', '/create', (request) {
-      request.response.write(json.stringify({
+      request.response.write(JSON.encode({
         'success': {'message': 'Package test_pkg 1.0.0 uploaded!'}
       }));
       request.response.close();
     });
 
     pub.shouldExit(0);
     expect(pub.remainingStdout(),
         completion(contains('Package test_pkg 1.0.0 uploaded!')));
   });
 }