done property Null safety
Return a future which is completed when the StreamSink is finished.
StreamSink fails with an error,
perhaps in response to adding events using add, addError or close,
the done future will complete with that error.
Otherwise, the returned future will complete when either:
- all events have been processed and the sink has been closed, or
- the sink has otherwise been stopped from handling more events (for example by canceling a stream subscription).
Future get done;