[perl #64018] Fix syntax error in sub declaration with invocant parameter

[via RT]

# New Ticket Created by  Cory Spencer 
# Please include the string:  [perl #64018]
# in the subject line of all future correspondence about this issue. 
# <URL: http://rt.perl.org/rt3/Ticket/Display.html?id=64018 >



[5:35pm] cspencer:rakudo: sub foo($x:) { say "x = $x" }; foo(2)
[5:35pm] p6eval:rakudo c26021: OUTPUT«error:imcc:syntax error,  
unexpected IDENTIFIER ('self')␤  in file 'EVAL_19' line 53␤»
[5:36pm] jnthn:fail
[5:36pm] jnthn:std: sub foo($x:) { say "x = $x" }; foo(2)
[5:36pm] amoc:it emits segmentation fault ;
[5:37pm] p6eval:std 25938: OUTPUT«ok 00:04 37m␤»
[5:37pm] jnthn:Damm, I can't steal an error from STD.pm. ;-)
[5:37pm] cspencer:heh :)
[5:37pm] jnthn:cspencer: File Rakudo bug, shouldn't be too hard to  
make that a better compile time error.