close app on device back button in flutter

What you need to do is first clear all path before going to login() screen.

try this:

onPressed:(){
    clearSession();

      //Navigator.popUntil(context, ModalRoute.withName('/'));
      Navigator.pop(context,true);// It worked for me instead of above line
      Navigator.pushReplacement(context, MaterialPageRoute(builder: (context) => Login()),);

},