_getChildChangeListener(slotName) {
      if (!this._childChangeListeners.has(slotName)) {
        this._childChangeListeners.set(slotName, this._onChildChange.bind(this, slotName));
      }
      return this._childChangeListeners.get(slotName);
    }
Returns a singleton event listener for the "change" event of a child in a given slot

@param slotName the name of the slot, where the child is
@returns {ChildChangeListener}
@private