Upgrading samples to flutter_lints, part 1 of n (#804)

testing_app/lib/screens/favorites.dart

@@ -13,7 +13,7 @@ class FavoritesPage extends StatelessWidget {
   Widget build(BuildContext context) {
     return Scaffold(
       appBar: AppBar(
-        title: Text('Favorites'),
+        title: const Text('Favorites'),
       ),
       body: Consumer<Favorites>(
         builder: (context, value, child) => value.items.isNotEmpty
@@ -23,7 +23,7 @@ class FavoritesPage extends StatelessWidget {
                 itemBuilder: (context, index) =>
                     FavoriteItemTile(value.items[index]),
               )
-            : Center(
+            : const Center(
                 child: Text('No favorites added.'),
               ),
       ),
@@ -52,11 +52,11 @@ class FavoriteItemTile extends StatelessWidget {
         ),
         trailing: IconButton(
           key: Key('remove_icon_$itemNo'),
-          icon: Icon(Icons.close),
+          icon: const Icon(Icons.close),
           onPressed: () {
             Provider.of<Favorites>(context, listen: false).remove(itemNo);
             ScaffoldMessenger.of(context).showSnackBar(
-              SnackBar(
+              const SnackBar(
                 content: Text('Removed from favorites.'),
                 duration: Duration(seconds: 1),
               ),