如何在没有包的情况下在Flutter中实现下拉列表？

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  final List<String> options = ['Option 1', 'Option 2', 'Option 3'];
  String selectedOption = 'Option 1';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Dropdown Example'),
        ),
        body: Center(
          child: DropdownButton<String>(
            value: selectedOption,
            onChanged: (String newValue) {
              selectedOption = newValue;
            },
            items: options.map<DropdownMenuItem<String>>((String value) {
              return DropdownMenuItem<String>(
                value: value,
                child: Text(value),
              );
            }).toList(),
          ),
        ),
      ),
    );
  }
}