Give default value for builder_tag in test options

tools/testing/dart/test_options.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 test_options_parser;
 
 import "dart:io";
 import "drt_updater.dart";
 import "test_suite.dart";
 
@@ skipping 372 matching lines @@
               ['--replay_from_file'],
               [],
               null),
           new _TestOptionSpecification(
               'builder_tag',
               'Machine specific options that is not captured by the regular '
               'test options. Used to be able to make sane updates to the '
               'status files.',
               ['--builder-tag'],
               [],
-              null),
+              ''),
           new _TestOptionSpecification(
               'vm_options',
               'Extra options to send to the vm when running',
               ['--vm-options'],
               [],
               null),];
   }
 
 
   /**
@@ skipping 425 matching lines @@
         return option;
       }
     }
     print('Unknown test option $name');
     exit(1);
   }
 
 
   List<_TestOptionSpecification> _options;
 }