[perl #123895] .gist for built-in Sub's prints bogus "uninitialized value $name" warning

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# New Ticket Created by  Sam S. 
# Please include the string:  [perl #123895]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=123895 >


Calling .gist on the Sub objects on of built-in subroutines/operators, causes a 
strange warning to be printed (in Rakudo 2015.01-156-g5ba2c86 with MoarVM 
2015.01-69-g5bbfd1a):

  > say &map.gist
  use of uninitialized value $name of type Any in string context  in block 
<unit> at <unknown file>:1
  
  sub map (Any |) { #`(Sub|44490920) ... }
  > say &postcircumfix:<[ ]>.gist
  use of uninitialized value $name of type Any in string context  in block 
<unit> at <unknown file>:1
  
  sub postcircumfix:<[ ]> (Any |) { #`(Sub|45073152) ... }

The problem does not occur with custom subs:

  >say (sub foo {}).gist
  sub foo () { #`(Sub|56973008) ... }