_invokerTemplate() {
    return html`
      <button
        type="button"
        @click="${this.__openCalendarOverlay}"
        id="${this.__invokerId}"
        aria-label="${this.msgLit('lion-input-datepicker:openDatepickerLabel')}"
        title="${this.msgLit('lion-input-datepicker:openDatepickerLabel')}"
      >
        ${this._invokerIconTemplate()}
      </button>
    `;
  }
Subclassers can replace this with their custom extension invoker,
like `<my-button><calendar-icon></calendar-icon></my-button>`