Future<T> next()

Returns an event that has been added to this server.

This method will return the first element in the first-in-first-out queue of events that have been added to this instance. If no events are available, this Future will complete when the next event is added.

Source 

Future<T> next() {
  if (_queue.isEmpty) {
    var c = new Completer<T>();
    _completerQueue.add(c);
    return c.future;
  }

  var val = _queue.removeAt(0);
  return new Future.value(val);
}