code: function notifyDrainListeners() {
          var notifiedSomebody = false;
          if (!!drainQueue.length) {
              // As we exhaust priority levels, notify the appropriate drain listeners.
              //
              var drainPriority = currentDrainPriority();
              while (+drainPriority === drainPriority && drainPriority > highWaterMark) {
                  pumpingPriority = drainPriority;
                  notifyCurrentDrainListener();
                  notifiedSomebody = true;
                  drainPriority = currentDrainPriority();
              }
          }
          return notifiedSomebody;
      }
docs: Notifies all drain listeners which are at a priority > highWaterMark. Returns whether or not any drain listeners were notified. This function sets pumpingPriority and reads highWaterMark. Note that it may call into user code which may call back into the scheduler.