- Object needle
needle occurs in the elements provided by this stream.
Compares each element of this stream to
needle using Object.==.
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 Object.== throws, the returned future is completed with that error, and processing stops.
@override AsObservableFuture<bool> contains(Object needle) => AsObservableFuture<bool>(_stream.contains(needle));