docs: function checkPort(port, cb) {
    var server = new net.Server();
    server.on('error', function (err) {
      server.removeAllListeners();
      cb(err);
    });
    server.listen(port, function () {
      server.on('close', function () {
        server.removeAllListeners();
        cb(null, port);
      });
      server.close();
    });
  }