Good point. I think there are use cases for both types of notifications.

One would be a request-response scenario (which maps well to a future and completer). The other scenario would be to listen to all push notification events from an isolate service. This would be something like the analysis server telling us that there was a new set of marker changes for the current context. So, on the `AnalysisService` interface, something like:

    Stream<AnalysisEvent> get onAnalysisEvent;