Updated deprecated widgets with new one (#666)

Author: adarsh-technocrat

testing_app/lib/screens/favorites.dart

@@ -55,7 +55,7 @@ class FavoriteItemTile extends StatelessWidget {
           icon: Icon(Icons.close),
           onPressed: () {
             Provider.of<Favorites>(context, listen: false).remove(itemNo);
-            Scaffold.of(context).showSnackBar(
+            ScaffoldMessenger.of(context).showSnackBar(
               SnackBar(
                 content: Text('Removed from favorites.'),
                 duration: Duration(seconds: 1),

testing_app/lib/screens/home.dart

@@ -67,7 +67,7 @@ class ItemTile extends StatelessWidget {
             !favoritesList.items.contains(itemNo)
                 ? favoritesList.add(itemNo)
                 : favoritesList.remove(itemNo);
-            Scaffold.of(context).showSnackBar(
+            ScaffoldMessenger.of(context).showSnackBar(
               SnackBar(
                 content: Text(favoritesList.items.contains(itemNo)
                     ? 'Added to favorites.'