_parseSlots() {
    const slots = this._slots = {};
    let n;
    while (n = this.firstChild) {
      const slotName = n.nodeType === 1 && n.getAttribute("slot") || "default";
      (slots[slotName] || (slots[slotName] = [])).push(n);
      this.removeChild(n);
    }
  }
Only called when shadowRoot is false