Returns a future that handles the onDone and onError callbacks.
This method overwrites the existing onDone and onError callbacks
with new ones that complete the returned future.
In case of an error the subscription will automatically cancel (even
when it was listening with cancelOnError set to false).
In case of a done event the future completes with the given
futureValue.
Source
Future asFuture([futureValue]) {
return null;
}