Example of how to use Dart's Completer

async_utils.dart

typedef CompleterBlock<T> = void Function(Completer<T> completer);

Future<T> completer<T>(final CompleterBlock<T> block) {
  assert(block != null);
  final completer = Completer<T>();
  block(completer);
  return completer.future;
}

example_usage.dart

import 'async_utils.dart';
import 'third_party_api.dart';

Future<void> doSomething() async {
  //bridge the gap using a Completer
  return completer((completer) {
      ThirdPartyApi().doSomethingThen(() {
        // the returned Future from doSomething will complete here when when calling completer.complete()
        completer.complete();
      });
  });
}

third_party_api.dart

class ThirdPartyApi {
  
  // lets say the third party API doesn't return a Future and instead 
  // takes a function that is run asynchronously
  void doSomethingThen(final Function() block) {
    Future.microtask(_longRunningTask).then((_) => block());
  }

  Future<void> _longRunningTask() {
    //TODO imagine long running op here
  }
}