TODO ------ * free datastructures when exiting * message passing, e.g. [ReceivePort](https://api.dartlang.org/stable/2.5.2/dart-isolate/ReceivePort-class.html), [SendPort](https://api.dartlang.org/stable/2.5.2/dart-isolate/SendPort-class.html) * transactions, atomicity guarantee etc.