I'm looking for something similar to takeWhile, but rather than performing the check before accepting the data, it performs the check after accepting the data.

I'm hitting this converting dart:html tests over to streams, in a number of the tests. The scenario is that they are listening to postMessage events until getting the one that they're looking for, then want to unsubscribe.

Alternatives are:
var complete = false;
window.onMessage.takeWhile((_) => !complete).listen((e) {
  if (e.data == 'hello') {
    complete = true;
  }
});

Which is ugly because it requires an extra event to unsubscribe and will get any errors before that event.

And:
var subscription = null;
subscription = window.onMessage.listen((e) {
  if (e.data == 'hello') {
    subscription.cancel();
  }
});
Which works, though the declaration of subscription is sort of ugly.