fix Flutter Japanese font problem in iOS12

fix_Flutter_Japanese_font_problem_in_iOS12.dart

ThemeData _buildTheme() {
  final ThemeData base = ThemeData.light();
  return base.copyWith(
      textTheme: base.textTheme.apply(fontFamily: 'Hiragino Kaku Gothic ProN'),
      primaryTextTheme:
          base.primaryTextTheme.apply(fontFamily: 'Hiragino Kaku Gothic ProN'),
      accentTextTheme:
          base.accentTextTheme.apply(fontFamily: 'Hiragino Kaku Gothic ProN'));
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: _buildTheme(),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}