如何使按钮的角仅在顶部变圆?

How can I make a button's corner only rounded on the top?

我正在尝试制作一个顶部有圆角而底部有普通角的按钮。我怎样才能做到这一点?

一般示例:

RaisedButton(
              onPressed: () {},
              shape: RoundedRectangleBorder(
                  borderRadius: BorderRadius.only(
                      topLeft: Radius.circular(15.0),
                      topRight: Radius.circular(15.0))),
              child: Text('Click Me!'),
              color: Colors.blueAccent,
              textColor: Colors.white,
            ),

根据 pskink 评论:

RaisedButton(
              onPressed: () {},
              shape: RoundedRectangleBorder(
                  borderRadius: BorderRadius.vertical(
                top: Radius.circular(15.0),
              )),
              child: Text('Click Me!'),
              color: Colors.blueAccent,
              textColor: Colors.white,
            ),