hello async https://dartpad.dartlang.org/a5d1305baf4bd3942a56c6b428790b88

main.dart

import 'dart:async';

void main() async {
  print(await f1(["hello"]));
}

Future<List<String>> f0(List<String> v) {
  return Future.value(v);
}

Future<List<String>> f1(List<String> v) {
  final Completer c = Completer();
  final f = f0(v);
  f.then(c.complete);
  return c.future.then((v) {
    return List<String>.from(v);
  });
}