Curved Canvas.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: MyPage(),
    );
  }
}

class MyPage extends StatefulWidget {
  @override
  _MyPageState createState() => _MyPageState();
}

class _MyPageState extends State<MyPage> {
  @override
  Widget build(BuildContext context) {
    var deviceWidth = MediaQuery.of(context).size.width;
    return Scaffold(
      body: SingleChildScrollView(
        child: Column(
          // mainAxisAlignment: MainAxisAlignment.start,
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            ClipPath(
                clipper: MyClipper(),
                child: Container(
                  padding: EdgeInsets.symmetric(horizontal: 10),
                  height: 300,
                  width: double.infinity,
                  decoration: BoxDecoration(
                    gradient: LinearGradient(
                      begin: Alignment.topRight,
                      end: Alignment.bottomLeft,
                      colors: [
                        Color(0xFF3383CD),
                        Color(0xFF11249F),
                      ],
                    ),
                  ),
                )),
          ],
        ),
      ),
    );
  }
}

class MyClipper extends CustomClipper<Path> {
  @override
  Path getClip(Size size) {
    var path = Path();
    path.lineTo(0, size.height - 80);
    path.quadraticBezierTo(
        size.width / 2, size.height, size.width, size.height - 80);
    path.lineTo(size.width, 0);
    path.close();
    return path;
  }

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