Converts the onData, on Done, and onError events into
objects that are passed into the downstream onData listener.
Notification object contains the Kind of event (OnData, onDone, or
OnError), and the item or error that was emitted. In the case of onDone,
no data is emitted as part of the
new Observable<int>.error(new Exception()) .materialize() .listen((i) => print(i)); // Prints onError Notification
Observable<Notification<T>> materialize() => transform(MaterializeStreamTransformer<T>());