function getFirstArgument(fn) {
    return (!hasKey(fn, 'toString') &&
      (/^[\s(]*function[^(]*\(([^\s,)]+)/.exec(fn) || 0)[1]) || '';
  }
Gets the name of the first argument from a function's source.

@private
@param {Function} fn The function.
@returns {String} The argument name.