Use case

We are using bottom sheet to display an asynchronous flow. This flow should not be dismissible until a success or error is shown.
Material design for Android and iOS already has an implementation of this

• Android: https://developer.android.com/reference/com/google/android/material/bottomsheet/BottomSheetBehavior (setCancelable)
• iOS: https://github.com/material-components/material-components-ios/blob/develop/components/BottomSheet/src/MDCBottomSheetController.h#L61

Proposal

let showModalBottomSheet takes in a variable isDismissable and pass it to _ModalBottomSheetRoute. This value will be returned instead for bool get barrierDismissible at
https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/material/bottom_sheet.dart#L363