code: function ListenerList (emitter, eventId) {
      this._count = 0; // count of "listeners"
      this._methods = null; // null, function, or array of functions
      this._objects = null; // null, context, or array of contexts
  
      if (debugLevel > 0) {
          this._emitter = emitter; // our parent EventEmitter
          this._eventId = eventId;
          this._counterMap = null; // key: listenerKey, value: count of listeners with same listenerKey
      }
  }
docs: --- Private helpers 
  Internal implementation of a "single eventID" emitter to a list of listeners.
  A ListenerList can be returned to outside world for "quick emit" purpose.
  
  @param {EventEmitter} emitter
  @param {string} eventId