Flutter不想导入json

我正在尝试关注此官方教程，以便向网址发出网络请求：https://flutter.io/cookbook/networking/fetch-data/

因此我在pubspec.yaml中添加了这些行：

dependencies:
  flutter:
    sdk: flutter

  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^0.1.2

  http: 0.11.3+17

  json_annotation: ^0.2.3

dev_dependencies:
  flutter_test:
    sdk: flutter

  build_runner: ^0.9.0

  json_serializable: ^0.5.4

编译时，我收到了这个错误 .

编译器消息：lib / main.dart:53:26：错误：找不到方法：'decode' . return Post.fromJson（json.decode（response.body））;

知道我做错了什么吗？这是我的代码：

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo2',
      theme: new ThemeData(
        primarySwatch: Colors.amber,
      ),
      home: new MyHomePage(title: 'Islam Essentiel'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class Post {
  final int userId;
  final int id;
  final String title;
  final String body;

  Post({this.userId, this.id, this.title, this.body});

  factory Post.fromJson(Map<String, dynamic> json) {
    return Post(
      userId: json['userId'],
      id: json['id'],
      title: json['title'],
      body: json['body'],
    );
  }
}

Future<Post> fetchPost() async {
  final response =
  await http.get('https://jsonplaceholder.typicode.com/posts/1');

  if (response.statusCode == 200) {
    // If server returns an OK response, parse the JSON
    return Post.fromJson(json.decode(response.body));
  } else {
    // If that response was not OK, throw an error.
    throw Exception('Failed to load post');
  }
}

class _MyHomePageState extends State<MyHomePage> {

  void _incrementCounter() {
    setState(() {


    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return new Scaffold(
      appBar: new AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: new Text(widget.title),
      ),
      body: new Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: new Column(
          // Column is also layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug paint" (press "p" in the console where you ran
          // "flutter run", or select "Toggle Debug Paint" from the Flutter tool
          // window in IntelliJ) to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Card(
              child: new Column(
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  const ListTile(
                    leading: const Icon(Icons.album),
                    title: const Text('al-khamis: 30. Muharram 1440'),
                    subtitle: const Text('20:51 - Icha.'),
                  ),
                  new ButtonTheme.bar( // make buttons use the appropriate styles for cards
                    child: new ButtonBar(
                      children: <Widget>[
                        new FlatButton(
                          child: const Text('MECQUE - DIRECTION'),
                          onPressed: () { /* ... */ },
                        ),
                      ],
                    ),
                  ),
                ],
              ),
            ),
          ],
        ),
      ),
      floatingActionButton: new FloatingActionButton(
    onPressed: fetchPost,
    tooltip: 'Increment',
    child: new Icon(Icons.add),
  ),
    );
  }
}