DART

flutter rounded bottom sheet

 void startAddNewMoodCheckin(BuildContext ctx) {
    showModalBottomSheet(
        shape: RoundedRectangleBorder(		//the rounded corner is created here
          borderRadius: BorderRadius.circular(10.0),
        ),
        context: ctx,
        builder: (_) {
          return Container(
            child: Text("This is a modal sheet"),
          );
        });
  }

Rounded Bottom Sheet Flutter

void _modalBottomSheetMenu(){
        showModalBottomSheet(
            context: context,
            builder: (builder){
              return new Container(
                height: 350.0,
                color: Colors.transparent, //could change this to Color(0xFF737373), 
                           //so you don't have to change MaterialApp canvasColor
                child: new Container(
                    decoration: new BoxDecoration(
                        color: Colors.white,
                        borderRadius: new BorderRadius.only(
                            topLeft: const Radius.circular(10.0),
                            topRight: const Radius.circular(10.0))),
                    child: new Center(
                      child: new Text("This is a modal sheet"),
                    )),
              );
            }
        );
      }