Add .old extension to silence analyzer

These examples haven't been ported to fn3 yet. Before this patch, they caused a
lot of noise in the analyzer output.

examples/game/test_performance.dart

@@ -37,7 +37,7 @@ main() async {
     title: 'Test Sprite Performance',
     theme: _theme,
     routes: {
-      '/': (NavigatorState navigator, RouteState route) {
+      '/': (RouteArguments args) {
         return new SpriteWidget(new TestPerformance());
       }
     }