[perl #123153] Assigning OUTER::OUTER::.keys() to a constant array generates an error

[Christian Bartolomaeus via RT]

This looks very similiar to Ticket #119521: 
https://rt.perl.org/Ticket/Display.html?id=119521

On Parrot we get a Null PMC access error with the same error message as in 
#119521:

$ perl6-p -e 'constant @SOMETHING = OUTER::OUTER::.keys();'
===SORRY!===
Null PMC access in get_attr_str()

If no one objects, I'm going to merge both tickets.