croupier: Fix warning about dealCards

I had forgotten to cast the game as a HeartsGame.

Change-Id: Id40fb9077f15c9e79139e13d43edef3af53d7637
diff --git a/lib/components/game.dart b/lib/components/game.dart
index e677ff0..3753a75 100644
--- a/lib/components/game.dart
+++ b/lib/components/game.dart
@@ -320,6 +320,8 @@
   }
 
   Widget showDeal() {
+    HeartsGame game = this.game as HeartsGame;
+
     return new Container(
         decoration: new BoxDecoration(backgroundColor: colors.Pink[500]),
         child: new Flex([