The semantic label used for a dismissible barrier.
If the barrier is dismissible, this label will be read out if accessibility tools (like VoiceOver on iOS) focus on the 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 this getter would ever start returning a different label, changedInternalState should be invoked so that the change can take effect.
final String barrierLabel