Add events dispatching for ActionScript framework Robotlegs
FileStream class has its own events which are dispatched when the file operation is successful or not.

http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/filesystem/FileStream.html#eventSummary

A possible refactor of the code is

protected function writeToFile():void{
    _destinationFile = File.applicationStorageDirectory.resolvePath(_newPath);

    var fs:FileStream = new FileStream();

    fs.open(_destinationFile, FileMode.WRITE);
    fs.writeBytes(_buffer, 0, _buffer.length);

    fs.addEventListener(Event.CLOSE, dispatchFileCreatedEvent);

    fs.close(); 

}


Thanks to krasimir of knowledge Robotlegs Community for his help in arriving at this answer.