passing variable - shows invalid variable ...

This is bizarre because I think the code looks fine. My best guess is that your dashboard view is getting returned somewhere else where $js_to_load isn't passed to it. Could it be something to do with the redirects, or perhaps another controller method is returning the dashboard view. That's where I'd start to look.

Alternatively you have unearthed a CI bug, but I think this seems unlikely. I might try renaming the variable to minimise this as a possibility.