main.dart

import 'dart:async';

import 'other.dart' show Error;

abstract class Wlan {
  Future<Error> method();
}

class WlanFuturize {
  final Wlan _inner;
  WlanFuturize(this._inner);
  void method(void callback(Error error)) =>
      _inner.method().then((Error _response) => callback(_response));
}

other.dart

class Error {}