code: function(opt_exe) {
    /** @private {string} */
    this.exe_ = opt_exe || io.findInPath(CHROMEDRIVER_EXE, true);
    if (!this.exe_) {
      throw Error(
          'The ChromeDriver could not be found on the current PATH. Please ' +
          'download the latest version of the ChromeDriver from ' +
          'http://chromedriver.storage.googleapis.com/index.html and ensure ' +
          'it can be found on your PATH.');
    }
  
    if (!fs.existsSync(this.exe_)) {
      throw Error('File does not exist: ' + this.exe_);
    }
  
    /** @private {!Array.<string>} */
    this.args_ = [];
    this.stdio_ = 'ignore';
  }
docs: Creates {@link remote.DriverService} instances that manage a ChromeDriver
  server.
  @param {string=} opt_exe Path to the server executable to use. If omitted,
  the builder will attempt to locate the chromedriver on the current
  PATH.
  @throws {Error} If provided executable does not exist, or the chromedriver
  cannot be found on the PATH.
  @constructor