_handlerIndex(h, type, handler) {
      for (let i = h ? h.length : 0; --i >= 0;) {
        if (h[i].type === type && (!handler || h[i].handler === handler)) {
          return i;
        }
      }
      return -1;
    }
Utility method for finding the array index of an event handler.
@param {Array} h - An array of registered event handlers.
@param {string} type - The event type.
@param {function} handler - The event handler instance to find.
@return {number} - The handler's array index or -1 if not registered.