_getSlotChangeListener(slotName) {
      if (!this._slotChangeListeners.has(slotName)) {
        this._slotChangeListeners.set(slotName, this._onSlotChange.bind(this, slotName));
      }
      return this._slotChangeListeners.get(slotName);
    }
Returns a singleton slotchange event listener that invalidates the component due to changes in the given slot

@param slotName the name of the slot, where the slot element (whose slotchange event we're listening to) is
@returns {SlotChangeListener}
@private