how to shape raised button to square code example

Example 1: rounded button flutter

FlatButton(
              color: Colors.red,
              shape: RoundedRectangleBorder(
                  borderRadius: BorderRadius.circular(18.0)),
              child: new Text('round button'),
              onPressed: () {},
            ),

Example 2: using shape property in flutter for circular corner

shape: RoundedRectangleBorder(
  borderRadius: BorderRadius.circular(18.0),
  side: BorderSide(color: Colors.red)
),