enable dark theme in flutter code example

Example 1: flutter dark theme

MaterialApp(
  darkTheme: ThemeData(
    brightness: Brightness.dark,
  ),

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'),
        );
      }
    );
  }
}