airでBitmapDataの保存
var pngSource:BitmapData = new BitmapData (stage.stageWidth, stage.stageHeight);
pngSource.draw(stage);
var ba:ByteArray = PNGEncoder.encode(pngSource);
var dir:File = File.desktopDirectory.resolvePath("AIR_Test/hoge");
dir.createDirectory();
var file:File = dir.resolvePath("test.png");
var fileStream:FileStream = new FileStream();
fileStream.open(file, FileMode.WRITE);
fileStream.writeBytes(ba);
fileStream.close();