Tag for the navigation bar's Hero widget if transitionBetweenRoutes is true.
Defaults to a common tag between all CupertinoNavigationBar and CupertinoSliverNavigationBar instances of the same Navigator. With the default tag, all navigation bars of the same navigator can transition between each other as long as there's only one navigation bar per route.
Cannot be null. To disable Hero transitions for this navigation bar, set transitionBetweenRoutes to false.
final Object heroTag;