anchor.diff:  }
   
   abstract class WebDriverBrowser extends Browser {
  -  static int _nextPort = 4444;
  -  final int _port = _nextPort++;
     WebDriver _driver;
   
     String get driverExecutable;
  
positive.code:       }
        if (_driver != null) break;
      }
    }
  
    @override
  
negative.code:   @override
    Future<bool> start(String url) async {
      _logEvent('Starting $this browser on: $url');
      var port = await _findUnusedPort();
      if (!await startBrowserProcess(
          driverExecutable, ['--port', '$port', ...driverArguments])) {
        return false;
      }
      await _createDriver(port);
      await _driver.get(url);
      try {
        _logEvent('Got version: ${await version}');
  