removeAllSubscriptions(eventType: ?string) {
    if (eventType === undefined) {
      this._subscriptionsForType = {};
    } else {
      delete this._subscriptionsForType[eventType];
    }
  }
Removes a bulk set of the subscriptions.

@param {?string} eventType - Optional name of the event type whose
  registered supscriptions to remove, if null remove all subscriptions.