private function saveFile():void{

        var myPattern:RegExp = / /g;

        var newFileName:String = fileName.text.replace('.txt','');

        if(newFileName.length > 1){

            var file:File = File.desktopDirectory.resolvePath("Files/" + newFileName.replace(myPattern,'_') + ".txt");

            var stream:FileStream = new FileStream()

            stream.open(file, FileMode.WRITE);

            var str:String = contents.text;

            str = str.replace(/\r/g, File.lineEnding);

            stream.writeUTFBytes(str);

            stream.close();

            fdg.directory = File.desktopDirectory.resolvePath("Files/");

            fileName.text = "";

            contents.text = "";

          } else {

              mx.controls.Alert.show("File name is required", "Error Saving File");

          }

    }

var file:File = File.desktopDirectory.resolvePath("Files/test.txt"); //тут путь поменять на нужный

var stream:FileStream = new FileStream()

stream.open(file, FileMode.WRITE);

var str:String = 'Приуэт';

stream.writeUTFBytes(str);

stream.close();