Star Border

import 'package:flutter/material.dart';

const SizedBox(
  width: 120,
  height: 60,
  child: Material(
    color: Colors.amber,
    shape: StarBorder(
      points: 8,
      innerRadiusRatio: 0.5,
      pointRounding: 0.1,
      side: BorderSide(
        color: Colors.purple,
        width: 2,
      ),
    ),
  ),
),