Collects the data of this stream in a
Set<T> and adds all elements of this stream to the set.
in the order they arrive.
When this stream ends, the returned future is completed with that set.
The returned set is the same type as returned by
If another type of set is needed, either use forEach to add each
element to the set, or use
toList().then((list) => new SomeOtherSet.from(list))
to create the set.
If this stream emits an error, the returned future is completed with that error, and processing stops.
@override AsObservableFuture<Set<T>> toSet() => AsObservableFuture<Set<T>>(_stream.toSet());