Called by the RestorationMixin to retrieve the information that this property wants to store in the restoration data.
The returned object must be serializable with the StandardMessageCodec and if it includes any collections, those should not be modified after they have been returned by this method.
The information returned by this method may be handed back to the property in a call to fromPrimitives at a later point in time (possibly after the application restarted) to restore the value that the property is currently wrapping.
@override Object? toPrimitives() => value?.millisecondsSinceEpoch;