Flutter - Awesome Shape

import 'package:flutter/material.dart';

class ShapePage extends StatefulWidget {
  @override
  _ShapePageState createState() => _ShapePageState();
}

class _ShapePageState extends State<ShapePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Awesome Shape'),
      ),
      body: Padding(
        padding: const EdgeInsets.all(32.0),
        child: Stack(
          children: <Widget>[
            Container(
              margin: EdgeInsets.only(bottom: 20.0),
              child: ClipPath(
                clipper: ArcClipper(),
                child: Container(
                    width: 200.0,
                    height: 50.0,
                    padding: EdgeInsets.all(8.0),
                    color: Colors.lightBlue,
                    child: Center(
                        child: Text(
                      'PROMO',
                      style: TextStyle(
                          color: Colors.white,
                          fontWeight: FontWeight.bold,
                          fontSize: 20.0),
                    ))),
              ),
            ),
            Positioned(
              bottom: 0.0,
              child: ClipPath(
                clipper: TriangleClipper(),
                child: Container(
                  width: 20.0,
                  height: 20.0,
                  color: Colors.blue[700],
                ),
              ),
            )
          ],
        ),
      ),
    );
  }
}

class ArcClipper extends CustomClipper<Path> {
  @override
  Path getClip(Size size) {
    var path = Path();
    path.lineTo(15.0, 0.0);

    var firstControlPoint = Offset(7.5, 2.0);
    var firstPoint = Offset(5.0, 5.0);
    path.quadraticBezierTo(firstControlPoint.dx, firstControlPoint.dy,
        firstPoint.dx, firstPoint.dy);

    var secondControlPoint = Offset(2.0, 7.5);
    var secondPoint = Offset(0.0, 15.0);
    path.quadraticBezierTo(secondControlPoint.dx, secondControlPoint.dy,
        secondPoint.dx, secondPoint.dy);

    path.lineTo(0.0, size.height);
    path.lineTo(size.width, size.height);
    path.lineTo(size.width - 20, size.height / 2);
    path.lineTo(size.width, 0.0);
    path.close();

    return path;
  }

  @override
  bool shouldReclip(CustomClipper<Path> oldClipper) => false;
}

class TriangleClipper extends CustomClipper<Path> {
  @override
  Path getClip(Size size) {
    var path = Path();
    path.lineTo(0.0, 0.0);
    path.lineTo(size.width, size.height);
    path.lineTo(size.width, 0.0);
    path.close();

    return path;
  }

  @override
  bool shouldReclip(CustomClipper<Path> oldClipper) => false;
}