The tag to apply to the button's Hero widget.
Defaults to a tag that matches other floating action buttons.
Set this to null explicitly if you don't want the floating action button to have a hero tag.
If this is not explicitly set, then there can only be one FloatingActionButton per route (that is, per screen), since otherwise there would be a tag conflict (multiple heroes on one route can't have the same tag). The material design specification recommends only using one floating action button per screen.
final Object heroTag