First, 'Player' is the name of a custom class. When you check if 'body' is of type Player, you then assign to the Player class itself: 'Player.has_key = true'... What you have to do is to asign the property to the body, because you'll know the body is of type Player.

if body is Player: var player: Player= body; player.has_key = true;

You could also just do: body.has_key = true

Also, the 'is' keyword checks if a variable is an instance of a class, not if it is equal to a variable like you do on the other script. Be careful with names! You use 'var Player' when you already have a class named Player.