Make it possible to check whether the receiver of a `SendPort` is alive.

[lrhn]

It's currently not possible to see whether sending on a SendPort is useful or it just gets lost in the void.
It can get lost either if the receiving isolate is dead, or just if the receive-port has stopped listening.

We should consider adding a Future<bool> get isAlive; (or similar) on SendPort which allows the user to check whether the receiver is still listening.
We should not distinguish whether the isolate is dead or the port is just closed.

Currently, all isolate communication is local, so there is no problem doing that. If we ever generalize to network-based communication, we'll need some way to handle lost messages and dropped connections anyway.