7034532: (fs) AssertionError when working directory is UNC

[Alan Bateman]

Mike Duigou wrote:
> The patch looks good to me. 
>
> The direct throwing of AssertionError is a bit odd. InternalError instead perhaps?
>
>
>   
I'm not sure whether AssertionError or InternalError is the better 
choice. In the Windows provider it throws AssertionError in the otehr 
"should not reach here" cases so I think I'll keep it consistent for now

-Alan