CustomClipper, answer for https://stackoverflow.com/questions/61544180/flutter-how-to-make-a-background-with-this-curve

main.dart

import 'package:flutter/material.dart';

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

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

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        children: [
          Expanded(child: Container()),
          Expanded(
            child: ClipPath(
              clipper: MyCustomClipper(),
              child: Container(
                decoration: BoxDecoration(
                  gradient: LinearGradient(
                    begin: Alignment.bottomRight,
                    end: Alignment.topLeft,
                    colors: [
                      const Color(0xFFFA709A),
                      const Color(0xFFFEC140),
                    ],
                  ),
                ),
              ),
            ),
          ),
        ],
      ),
    );
  }
}

class MyCustomClipper extends CustomClipper<Path> {
  @override
  Path getClip(Size size) {
    Path path = Path()
      ..moveTo(size.width, 0)
      ..lineTo(size.width, size.height)
      ..lineTo(0, size.height)
      ..lineTo(0, 60)
      ..arcToPoint(
        Offset(30, 30),
        radius: Radius.circular(30),
      )
      ..lineTo(size.width - 30, 30)
      ..arcToPoint(Offset(size.width, 0),
          radius: Radius.circular(30), clockwise: false);
    return path;
  }

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