StreamGroup<T>  constructor 
      
      StreamGroup<T> ()
     
    
Creates a new stream group where stream is single-subscriber.
Implementation
StreamGroup() {
  _controller = StreamController<T>(
      onListen: _onListen,
      onPause: _onPause,
      onResume: _onResume,
      onCancel: _onCancel,
      sync: true);
}