code: function (eventName, arg1, arg2) {
        var callbacks = this._eventCallbacks[eventName];
        if (callbacks) {
          for (var i = 0; i < callbacks.length; i++) {
            callbacks[i](arg1, arg2);
          }
        }
      }
docs: Trigger an event. Supports up to two arguments. Designed around
  triggering transition events from one run loop instance to the
  next, which requires an argument for the first instance and then
  an argument for the next instance.
  @private
  @method _trigger
  @param {String} eventName
  @param {any} arg1
  @param {any} arg2