如何在下面的代码中使自定义FlatButton的边框“更粗”？

import 'package:flutter/material.dart';

class CustomFlatButton extends FlatButton {
  CustomFlatButton({
    Key key,
    @required VoidCallback onPressed,
    ValueChanged<bool> onHighlightChanged,
    ButtonTextTheme textTheme,
    Color textColor,
    Color disabledTextColor,
    Color color,
    Color disabledColor,
    Color highlightColor,
    Color splashColor,
    Brightness colorBrightness,
    EdgeInsetsGeometry padding,
    ShapeBorder shape,
    Clip clipBehavior = Clip.none,
    FocusNode focusNode,
    bool autofocus = false,
    MaterialTapTargetSize materialTapTargetSize,
    @required Widget child,
  }) : super(
          key: key,
          onPressed: onPressed,
          onHighlightChanged: onHighlightChanged,
          textTheme: textTheme,
          textColor: textColor,
          disabledTextColor: disabledTextColor,
          color: color,
          disabledColor: disabledColor,
          highlightColor: highlightColor,
          splashColor: splashColor,
          colorBrightness: colorBrightness,
          padding: padding,
          shape: shape,
          clipBehavior: clipBehavior,
          focusNode: focusNode,
          autofocus: autofocus,
          materialTapTargetSize: materialTapTargetSize,
          child: child,
        );
}

class CustomFlatButton extends FlatButton {
  // ...

  @override
  Widget build(BuildContext context) {
    return Container(
      decoration: BoxDecoration(
        border: Border.all(
          color: Colors.black, // 设置边框颜色
          width: 2.0, // 设置边框宽度
        ),
        borderRadius: BorderRadius.circular(8.0), // 设置边框圆角
      ),
      child: super.build(context),
    );
  }
}

CustomFlatButton(
  onPressed: () {
    // 按钮点击事件处理
  },
  child: Text('按钮'),
),