function before(decorator, fn) {
  return function () {
    decorator.apply(null, arguments);
    fn.apply(null, arguments);
  };
}
Function decorating one function with another that will be called before the
decorated one.

@param  {function} decorator - The decorating function.
@param  {function} fn        - The function to decorate.
@return {function}           - The decorated function.