函数中的dart上下文未定义名称‘Navigator.pushReplacement’

import 'package:flutter/material.dart';

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

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

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Home'),
      ),
      body: Center(
        child: RaisedButton(
          child: Text('Go to Next Page'),
          onPressed: () {
            Navigator.pushReplacement(
              context,
              MaterialPageRoute(builder: (context) => NextPage()),
            );
          },
        ),
      ),
    );
  }
}

class NextPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Next Page'),
      ),
      body: Center(
        child: Text('This is the next page.'),
      ),
    );
  }
}