Make kPestoRecipes a constant (#7886)

Someone on stack overflow was mutating the list of recipes and getting
confused about why the UI didn't redraw. Making kPestoRecipes a constant
might help avoid that confusion.

examples/flutter_gallery/lib/demo/pesto_demo.dart

@@ -551,7 +551,7 @@ class RecipeStep {
   final String description;
 }
 
-final List<Recipe> kPestoRecipes = <Recipe>[
+const List<Recipe> kPestoRecipes = const <Recipe>[
   const Recipe(
     name: 'Pesto Bruschetta',
     author: 'Peter Carlsson',