Go like defer in Dart

defer.dart

import 'dart:async' show Future, Zone, scheduleMicrotask;

final Object _deferredPushKey = Object();

/// Defers the given [callback] function to be executed after the current zone.
/// The callback will be executed in the reverse order of their registration.
void defer(void Function() callback) {
  final push = Zone.current[_deferredPushKey];
  if (push is void Function(void Function() callback)) {
    push(callback);
  } else {
    throw StateError('defer() can only be called within runWithDefer()');
  }
}

/// Runs the given [body] function in a new zone with deferred execution support.
T runWithDefer<T>(T Function() body, {bool microtask = false}) {
  void Function()? stack;

  void push(void Function() callback) {
    final prev = stack;
    stack = () {
      try {
        callback();
      } finally {
        prev?.call();
      }
    };
  }

  void execute() {
    final call = stack;
    if (call == null) {
      return;
    } else if (microtask) {
      scheduleMicrotask(call);
    } else {
      call();
    }
  }

  return Zone.current.fork(specification: null, zoneValues: <Object?, Object?>{_deferredPushKey: push}).run<T>(() {
    var handled = false;
    try {
      final value = body();
      assert(value is! Stream<Object?>, 'Stream is not supported by runWithDefer()');
      if (value is Future<Object?>) {
        handled = true;
        return value..whenComplete(execute);
      }
      return value;
    } on Object {
      rethrow;
    } finally {
      if (!handled) execute();
    }
  });
}

main.dart

import 'dart:async';

import 'defer.dart';

class Client {
  Client() {
    print('Client created');
  }

  Future<void> fetch(Object url) =>
      Future.delayed(const Duration(milliseconds: 250), () => print('Client fetched $url'));

  void close() => print('Client closed');
}

void main(List<String> arguments) => runWithDefer<void>(() async {
  final client = Client();
  defer(() => print('Program finished'));
  defer(client.close);
  await client.fetch('http://a.ru');
  await client.fetch('https://b.com');
});