Revert "Use native async/await support in pub."

sdk/lib/_internal/pub_generated/test/lish/upload_form_is_missing_url_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:convert';
 
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:scheduled_test/scheduled_server.dart';
 
 import '../descriptor.dart' as d;
@@ skipping 17 matching lines @@
         'field2': 'value2'
       }
     };
 
     handleUploadForm(server, body);
     pub.stderr.expect('Invalid server response:');
     pub.stderr.expect(JSON.encode(body));
     pub.shouldExit(1);
   });
 }