code: function processSlotOutlet (el) {
    if (el.tag === 'slot') {
      el.slotName = getBindingAttr(el, 'name');
      if (el.key) {
        warn$2(
          "`key` does not work on <slot> because slots are abstract outlets " +
          "and can possibly expand into multiple elements. " +
          "Use the key on a wrapping element instead.",
          getRawBindingAttr(el, 'key')
        );
      }
    }
  }
docs: handle <slot/> outlets