A small hack to apply circular radius to an uniform border.

hack_no_uniform_border_radius.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: ThirdScreen(),
        ),
      ),
    );
  }
}

class ThirdScreen extends StatefulWidget {
  @override
  _ThirdScreenState createState() => _ThirdScreenState();
}

class _ThirdScreenState extends State<ThirdScreen> {
  TextEditingController _controller = new TextEditingController(text: "0");
  @override
  Widget build(BuildContext context) {
    double scale = 1;
    Radius radius = Radius.circular(5.0 * scale);
    Color inputColor = darkBlue;
    Color inputBorderColor = Colors.grey;
    Color editIconColor = Colors.orangeAccent;
    return Row(
      mainAxisSize: MainAxisSize.min,
      children: <Widget>[
        Container(
          height: 24 * scale,
          width: 50 * scale,
          decoration: BoxDecoration(
            color: inputBorderColor,
            borderRadius: BorderRadius.horizontal(left: radius),
          ),
          child: Container(
            margin: EdgeInsets.fromLTRB(scale, scale, 0, scale),
            decoration: BoxDecoration(
              color: inputColor,
              borderRadius: BorderRadius.horizontal(left: radius),
            ),
            child: Row(
              children: <Widget>[
                Flexible(
                  child: TextFormField(
                    controller: _controller,
                    style: TextStyle(fontSize: 14 * scale),
                    textAlign: TextAlign.right,
                    maxLength: 3,
                    maxLines: 1,
                    keyboardType: TextInputType.number,
                    decoration: InputDecoration(
                      focusedBorder: InputBorder.none,
                      enabledBorder: InputBorder.none,
                      counterText: '',
                    ),
                  ),
                ),
                Text(
                  ' % ',
                  style: TextStyle(fontSize: 14 * scale),
                ),
              ],
            ),
          ),
        ),
        Container(
          width: 24 * scale,
          height: 24 * scale,
          decoration: BoxDecoration(
            border: Border.all(color: editIconColor, width: scale),
            borderRadius: BorderRadius.horizontal(right: radius),
          ),
          child: IconButton(
            onPressed: null,
            iconSize: 22 * scale,
            padding: EdgeInsets.all(0),
            icon: Icon(
              Icons.edit,
              color: editIconColor,
            ),
          ),
        )
      ],
    );
  }
}