Whether this object is in the process of being disposed.
Deprecated: 1.9.0 To be removed: 2.0.0
isOrWillBeDisposed should be used instead.
This getter is useful for Disposable tests, but for public consumption, isOrWillBeDisposed is more useful because it remains true throughout the "awaiting disposal", "disposing", and "disposed" states.
@deprecated bool get isDisposing => _state == DisposableState.disposing;