Remove usage of dart:json.

sdk/lib/_internal/pub/test/lish/utils.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.
 
 library lish.utils;
 
+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';
 
 void handleUploadForm(ScheduledServer server, [Map body]) {
   server.handle('GET', '/api/packages/versions/new', (request) {
     return server.url.then((url) {
       expect(request.headers.value('authorization'),
           equals('Bearer access token'));
 
       if (body == null) {
         body = {
           'url': url.resolve('/upload').toString(),
           'fields': {
             'field1': 'value1',
             'field2': 'value2'
           }
         };
       }
 
       request.response.headers.contentType =
           new ContentType("application", "json");
-      request.response.write(json.stringify(body));
+      request.response.write(JSON.encode(body));
       request.response.close();
     });
   });
 }
 
 void handleUpload(ScheduledServer server) {
   server.handle('POST', '/upload', (request) {
     // TODO(nweiz): Once a multipart/form-data parser in Dart exists, validate
     // that the request body is correctly formatted. See issue 6952.
     return drainStream(request).then((_) {
       return server.url;
     }).then((url) {
       request.response.statusCode = 302;
       request.response.headers.set(
           'location', url.resolve('/create').toString());
       request.response.close();
     });
   });
 }