Better check if the page is in dashboard

Detail: http://www.concrete5.org/developers/bugs/5-6-3-1/using-dashboard-as-the-first-word-in-a-page-url-disables-admined/
I wonder if there's anything wrong with these lines: https://github.com/concrete5/concrete5/blob/master/web/concrete/helpers/concrete/dashboard.php#L51-L52

If there's a problem, shouldn't we fix it there instead of passing `$page` into `inDashboard`?