dispatchEvent method 
    
      
void
dispatchEvent(
 - Event event
 
)
      
     
    
    
  Implementation
  void dispatchEvent(Event event) {
  if (_listeners == null || _listeners!.isEmpty) return;
  var listeners = _listeners!;
  var listenerArray = listeners[event.type];
  // print("dispatchEvent event: ${event.type} ");
  if (listenerArray != null) {
    event.target = this;
    // Make a copy, in case listeners are removed while iterating.
    var array = listenerArray.sublist(0);
    for (var i = 0, l = array.length; i < l; i++) {
      Function _fn = array[i];
      _fn(event);
    }
    event.target = null;
  }
}