download_large_file.as

var urlString:String = "http://example.com/myfile.zip";
var urlReq:URLRequest = new URLRequest(urlString);
private var urlStream:URLStream = new URLStream();
private var fileStream:FileStream = new FileStream();

urlStream.addEventListener(Event.COMPLETE, loaded);
urlStream.addEventListener(ProgressEvent.PROGRESS, writeFile);

var file:File = File.desktopDirectory.resolvePath("myfile.zip");
fileStream.openAsync(file, FileMode.WRITE);

urlStream.load(urlReq);

function writeFile(event:ProgressEvent):void {
  // only write every 50k or so downloaded
  if (urlStream.bytesAvailable > 51200) {
    // Read the buffer into a ByteArray and write it to disk
    var data:ByteArray = new ByteArray();
    urlStream.readBytes(data, 0, urlStream.bytesAvailable);
    fileStream.writeBytes(data, 0, data.length);
  }
}

function loaded(event:Event):void {
  // Write any remaining data to the file before closing it
  var data:ByteArray = new ByteArray();
  urlStream.readBytes(data, 0, urlStream.bytesAvailable);
  fileStream.writeBytes(data, 0, data.length);
  fileStream.close();
}