how to add dark mode to flutter app code example

Example 1: how can i add dark mode quickly in flutter

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

class MyApp extends StatelessWidget {
  final _notifier = ValueNotifier<ThemeModel>(ThemeModel(ThemeMode.light));

  @override
  Widget build(BuildContext context) {
    return ValueListenableBuilder<ThemeModel>(
      valueListenable: _notifier,
      builder: (_, model, __) {
        final mode = model.mode;
        return MaterialApp(
          theme: ThemeData.light(), // Provide light theme.
          darkTheme: ThemeData.dark(), // Provide dark theme.
          themeMode: mode, // Decides which theme to show.
          home: Scaffold(
            appBar: AppBar(title: Text('Light/Dark Theme')),
            body: RaisedButton(
              onPressed: () => _notifier.value = ThemeModel(mode == ThemeMode.light ? ThemeMode.dark : ThemeMode.light),
              child: Text('Toggle Theme'),
            ),
          ),
        );
      },
    );
  }
}

class ThemeModel with ChangeNotifier {
  final ThemeMode _mode;
  ThemeMode get mode => _mode;

  ThemeModel(this._mode);
}

Example 2: dark mode in flutter packages

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new DynamicTheme(
      defaultBrightness: Brightness.light,
      data: (brightness) => new ThemeData(
        primarySwatch: Colors.indigo,
        brightness: brightness,
      ),
      themedWidgetBuilder: (context, theme) {
        return new MaterialApp(
          title: 'Flutter Demo',
          theme: theme,
          home: new MyHomePage(title: 'Flutter Demo Home Page'),
        );
      }
    );
  }
}