如何在AlertDialog - Flutter网站中将CircularProgressIndicator居中

import 'package:flutter/material.dart';

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

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

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('AlertDialog - Flutter'),
      ),
      body: Center(
        child: RaisedButton(
          child: Text('显示对话框'),
          onPressed: () {
            showDialog(
              context: context,
              builder: (BuildContext context) {
                return AlertDialog(
                  title: Text('加载中'),
                  content: Container(
                    child: Center(
                      child: CircularProgressIndicator(),
                    ),
                  ),
                  actions: <Widget>[
                    FlatButton(
                      child: Text('关闭'),
                      onPressed: () {
                        Navigator.of(context).pop();
                      },
                    ),
                  ],
                );
              },
            );
          },
        ),
      ),
    );
  }
}