Semi Rounded Button in Flutter.

semi_rounded_button.dart

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:meta/meta.dart';

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      theme: new ThemeData.dark(),
      home: new Material(
        color: const Color(0xff679790),
        child: new Center(
          child: new SizedBox(
            width: 200.0,
            height: 70.0,
            child: new SemiRoundedBorderButton(
              borderSide: const BorderSide(color: const Color(0xffDDE9E8), width: 2.0),
              radius: const Radius.circular(8.0),
              background: const Color(0xFF83ADA8),
              child: new Center(child: new Text('Testing')),
            ),
          ),
        ),
      ),
    );
  }
}

class SemiRoundedBorderButton extends StatelessWidget {
  final BorderSide borderSide;
  final Radius radius;
  final Color background;
  final Widget child;

  const SemiRoundedBorderButton({
    Key key,
    @required this.borderSide,
    @required this.radius,
    @required this.background,
    @required this.child,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return new ClipRect(
      clipper: new SemiRoundedBorderClipper(borderSide.width + 1.0),
      child: new DecoratedBox(
        decoration: new ShapeDecoration(
          color: background,
          shape: new RoundedRectangleBorder(
            side: borderSide,
            borderRadius: new BorderRadius.only(
              topLeft: radius,
              topRight: radius,
            ),
          ),
        ),
        child: child,
      ),
    );
  }
}

class SemiRoundedBorderClipper extends CustomClipper<Rect> {
  final double borderStrokeWidth;

  const SemiRoundedBorderClipper(this.borderStrokeWidth);

  @override
  Rect getClip(Size size) {
    return new Rect.fromLTRB(0.0, 0.0, size.width, size.height - borderStrokeWidth);
  }

  @override
  bool shouldReclip(CustomClipper<Rect> oldClipper) {
    if (oldClipper.runtimeType != SemiRoundedBorderClipper) return true;
    return (oldClipper as SemiRoundedBorderClipper).borderStrokeWidth != borderStrokeWidth;
  }
}