Use case

Hello,

I'm loving the look of the CupertinoSheetRoute and its convenience method showCupertinoSheet however currently it seems that it isn't possible to customize the top gap which is restricting it's usage somewhat as a viable alternative to the material version showModalBottomSheet.

Currently it seems the top gap is hardcoded as: const double _kTopGapRatio = 0.08;

Proposal

I propose that this top gap valuable becomes an optional property, with the default value being kept as it is.

This could be implemented in the showCupertinoModalPopup via the following example:

      await showCupertinoSheet(
          context: context, 
          topGap: 2 
          pageBuilder: (BuildContext context) => Text("Example text") 
      );