main() async {
  List letters = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"];
  File file = new File("Letters.txt");
  for (int i = 0; i < 10; i++) {
    await file.writeAsString("${letters[i]}", mode: FileMode.append);
  }
}