Acts as a container for multiple subscriptions that can be canceled at once
e.g. view subscriptions in Flutter that need to be canceled on view disposal
A class that encapsulates the NotificationKind of event, value of the event in case of
onData, or the Error in the case of onError.
A container object that wraps the NotificationKind of event (OnData, OnDone, OnError),
and the item or error that was emitted. In the case of onDone, no data is
emitted as part of the StreamNotification.
