Whether you can dismiss this route by tapping the modal barrier.
The modal barrier is the scrim that is rendered behind each route, which generally prevents the user from interacting with the route below the current route, and normally partially obscures such routes.
For example, when a dialog is on the screen, the page below the dialog is usually darkened by the modal barrier.
If barrierDismissible is false, then tapping the barrier has no effect.
If this getter would ever start returning a different value, changedInternalState should be invoked so that the change can take effect.
final bool barrierDismissible