'closure_1 as closure_2' problem

main.dart

import 'dart:isolate';

void main() async {
  final toMainIsolate = ReceivePort();
  final stream = toMainIsolate;

  void fn(ITargetAction action) => action.handleAction();

  final worker = Worker<ITargetAction>(
    port: toMainIsolate.sendPort,
    action: TargetActionImpl(),
  );
  final isolate = await worker.spawn();

  final sendPort = await stream.first as SendPort;

  sendPort.send(fn);
}

abstract interface class ITargetAction {
  void handleAction();
}

class TargetActionImpl implements ITargetAction {
  @override
  void handleAction() => print('HOORAY');
}

class Worker<T extends ITargetAction> {
  Worker({
    required SendPort port,
    required T action,
  })  : _port = port,
        _action = action;

  final SendPort _port;
  final T _action;

  static void _workerEndpoint<T extends ITargetAction>(Worker<T> worker) {
    worker();
  }

  Future<Isolate> spawn() => Isolate.spawn<Worker<ITargetAction>>(
        _workerEndpoint,
        this,
      );

  void send(void Function(T) handler) => _port.send(handler);

  void _receiveAction(void Function(T) handleAction) => handleAction(_action);

  void call() => _port.send((ReceivePort()
        ..listen((data) {
          if (data is! void Function(T)) return;
          _receiveAction(data);
        }))
      .sendPort);
}