function olderVueCompat (options) {
  if (Vue.version.startsWith('2.4')) return options
  return {
    props: {
      ...options.props,
      passedProps: {
        ...options.attrs,
        ...options.on,
      },
    },
  }
}
In Vue < 2.4.0, there's no $attrs or $listeners. We have to pass props through a registered
prop `passedProps`.