The curve that is used for animating the modal barrier in and out.
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.
While the route is animating into position, the color is animated from transparent to the specified barrierColor.
If this getter would ever start returning a different curve, changedInternalState should be invoked so that the change can take effect.
It defaults to Curves.ease.
Curve get barrierCurve => Curves.ease;