uncaught gist

main.dart

import 'dart:async';

void main() async {
   String? callbacker(void Function(Object? arg) cb) {
      cb(null); // indicates failure
    }
    Object? error;
    try {
      await _futurize(callbacker);
    } catch (err) {
      error = err;
    }
  print(error);
}

typedef _Callback<T> = void Function(T result);
typedef _Callbacker<T> = String? Function(_Callback<T?> callback);

// This is an exact copy of the function defined in painting.dart. If you change either
// then you must change both.
Future<T> _futurize<T>(_Callbacker<T> callbacker) {
  final Completer<T> completer = Completer<T>.sync();
  // If the callback synchronously throws an error, then synchronously
  // rethrow that error instead of adding it to the completer. This
  // prevents the Zone from receiving an uncaught exception.
  bool sync = true;
  final String? error = callbacker((T? t) {
    if (t == null) {
      // remove the `if (sync)` condition for a demonstration
      if (sync) {
        throw Exception('operation failed');
      } else {
        completer.completeError(Exception('operation failed'));
      }
    } else {
      completer.complete(t);
    }
  });
  sync = false;
  if (error != null)
    throw Exception(error);
  return completer.future;
}