title: Locale from web app is always 'en' Flutter
body: <p>I have my app that is correctly getting device locale from Android and iOS, but not from web. I get the web locale language code as <code>ui.window.locale.languageCode</code> using <code>ui.dart</code> package. In my Mac I set the system language to Italian, time zone is set to Italy and Location services set to enabled but when I run my app on Chrome it always return 'en_'.
  Is there something I am missing to do?
  This is the code in main() :</p>
  
  <pre><code>class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
  //    Locale myLocale = Localizations.localeOf(context);
      return MaterialApp(
          title: '',
          color: Colors.red,
          localizationsDelegates: [
            const AppLocalizationsDelegate(),
            GlobalMaterialLocalizations.delegate,
            GlobalCupertinoLocalizations.delegate,
            GlobalWidgetsLocalizations.delegate,
          ],
          supportedLocales: [
            const Locale('en', 'US'),
            const Locale('it', 'IT')
  //        const Locale('es', 'ES'),
          ],
          localeResolutionCallback:
              (Locale locale, Iterable&lt;Locale&gt; supportedLocales) {
            for (Locale supportedLocale in supportedLocales) {
              if (kIsWeb) {
                Locale webLocale = Locale(ui.window.locale.languageCode, '');
                print('system locale is ${webLocale}');
                return webLocale;
              } else if (supportedLocale.languageCode == locale.languageCode ||
                  supportedLocale.countryCode == locale.countryCode) {
                print('device Locale is $locale');
                return supportedLocale;
              }
            }
            return supportedLocales.first;
          },
  ...
  </code></pre>