Alternative to elixir `mix.scriptsIn` in laravel-mix

Laravel 5.4 introduces laravel-mix.
With elixir I had one task that loads all vendor files into one specific:

elixir(function (mix) {
  mix.stylesIn('public/vendor/css', 'public/css/vendor.css');

  mix.scriptsIn('public/vendor/js', 'public/js/vendor.js');
}

Is possible to do the same with laravel-mix?

Using mix.js('/path/to/folder', 'public/js/vendor.js') i get an error:

Error: EISDIR: illegal operation on a directory

via Victor