needle occurs in the elements provided by this stream.
Compares each element of this stream to
If an equal element is found, the returned future is completed with
If this stream ends without finding a match, the future is completed with
If this stream emits an error, or the call to
the returned future is completed with that error,
and processing stops.
@override AsObservableFuture<bool> contains(Object needle) => AsObservableFuture<bool>(_stream.contains(needle));