Fix examples/game to work after Hixie's grand rename.

TBR=viktorl@google.com

Review URL: https://codereview.chromium.org/1158273009

examples/game/lib/sprites.dart

@@ -5,7 +5,7 @@ import 'dart:math' as Math;
 import 'package:vector_math/vector_math_64.dart';
 import 'package:sky/framework/app.dart';
 import 'package:sky/framework/rendering/box.dart';
-import 'package:sky/framework/rendering/node.dart';
+import 'package:sky/framework/rendering/object.dart';
 import 'package:sky/framework/scheduler.dart' as scheduler;
 import 'package:sky/framework/net/image_cache.dart' as image_cache;