Moving size from BaseGame to Game

doc/examples/with_widgets_overlay/lib/example_game.dart

@@ -10,6 +10,11 @@ class ExampleGame extends Game with TapDetector {
   @override
   void update(double dt) {}
 
+  @override
+  Future<void> onLoad() async {
+    print('game loaded');
+  }
+
   @override
   void render(Canvas canvas) {
     canvas.drawRect(