close method Null safety
Tells the stream sink that no further streams will be added.
This allows the stream sink to complete any remaining work and release resources that are no longer needed
Returns a future which is completed when the stream sink has shut down.
If cleaning up can fail, the error may be reported in the returned future,
otherwise it completes with
Returns the same future as done.
The stream sink may close before the close method is called, either due
to an error or because it is itself providing events to someone who has
stopped listening. In that case, the done future is completed first,
close method will return the
done future when called.