Discards all data on the stream, but signals when it's done or an error occurred.
When subscribing using drain, cancelOnError will be true. This means
that the future will complete with the first error on the stream and then
cancel the subscription.
In case of a done event the future completes with the given
futureValue.
Source
Future<E> drain<E>([E futureValue]) =>
listen(null, cancelOnError: true).asFuture<E>(futureValue);