A validator of type Future

Hello,

I'm trying to use Formz to validate a phonenumber using the https://pub.dev/packages/libphonenumber library.

But libphonenumber's validation function returns a Future, so I was unable to use it in the validator() function of Formz

import 'package:formz/formz.dart';
import 'package:libphonenumber/libphonenumber.dart';

enum PhoneNumberValidationError { invalid }

class PhoneNumber extends FormzInput<String, PhoneNumberValidationError> {
  const PhoneNumber.pure() : super.pure('');

  const PhoneNumber.dirty([String value = '']) : super.dirty(value);

  static final _phoneNumberRegex = RegExp(r'^[+]{1}[0-9]{1,4}[-\s\./0-9]*$');

  @override
  PhoneNumberValidationError validator(String value) {
    return PhoneNumberUtil.isValidPhoneNumber(phoneNumber: "", isoCode: 'US').then((value){
      return null;
    });
  }
}

error: A value of type 'Future<Null>' can't be returned from method 'validator' because it has a return type of 'PhoneNumberValidationError'. (return_of_invalid_type at [datoraid] lib/models/phone_number.dart:16)

Read the FormzInput validator contract and the example in lib/models/phone_number.dart. Confirm how the synchronous return type conflicts with libphonenumber's Future, then define the desired async-validation API and its completion criteria with maintainers.