code: function webdriverDownload(done) {
    const platform = os.platform();
    const destDir = path.join(__dirname, '.selenium');
    const chromeDriverUrl = (() => {
      const filePath = platform === 'win32' ?
        '/2.38/chromedriver_win32.zip' :
        `/2.38/chromedriver_${platform === 'darwin' ? 'mac' : 'linux'}64.zip`;
      return `http://chromedriver.storage.googleapis.com${filePath}`;
    })();
  
    // Only download once.
    if (fs.existsSync(destDir + '/chromedriver') || fs.existsSync(destDir + '/chromedriver.exe')) {
      done();
    } else {
      const selenium = $.download('https://selenium-release.storage.googleapis.com/3.12/selenium-server-standalone-3.12.0.jar')
        .pipe(gulp.dest(destDir));
    
      const chromedriver = $.download(chromeDriverUrl)
        .pipe($.unzip())
        .pipe($.chmod(755))
        .pipe(gulp.dest(destDir));
    
      return mergeStream(selenium, chromedriver);
    }
  }
docs: ////////////////////////////////////// webdriver-download (overrides parent definition) //////////////////////////////////////