Enable Material 3 in testing_app (#1628)

Co-authored-by: Brett Morgan <brettmorgan@google.com>

testing_app/lib/main.dart

@@ -40,8 +40,9 @@ class TestingApp extends StatelessWidget {
       child: MaterialApp.router(
         title: 'Testing Sample',
         theme: ThemeData(
-          primarySwatch: Colors.blue,
+          colorSchemeSeed: Colors.blue,
           visualDensity: VisualDensity.adaptivePlatformDensity,
+          useMaterial3: true,
         ),
         routerConfig: router(),
       ),

testing_app/lib/screens/home.dart

@@ -20,7 +20,6 @@ class HomePage extends StatelessWidget {
         title: const Text('Testing Sample'),
         actions: [
           TextButton.icon(
-            style: TextButton.styleFrom(foregroundColor: Colors.white),
             onPressed: () {
               context.go(FavoritesPage.fullPath);
             },