[perl #104980] natively typed attributes and BUILD methods don't mix

[Christian Bartolomaeus via RT]

As a status update: This currently fails with:

$ perl6-m -e 'class A { has str $.x; method BUILD(:$!x) { } }; say 
A.new(:x<foo>).x'
Cannot modify an immutable str
  in method BUILD at -e:1
  in method BUILDALL at src/gen/m-CORE.setting:983
  in method bless at src/gen/m-CORE.setting:966
  in method new at src/gen/m-CORE.setting:952
  in block <unit> at -e:1

$ perl6-j -e 'class A { has str $.x; method BUILD(:$!x) { } }; say 
A.new(:x<foo>).x'
java.lang.RuntimeException: Cannot access a native attribute as a reference 
attribute
  in method BUILD at -e:1
  in method BUILDALL at gen/jvm/CORE.setting:965
  in method bless at gen/jvm/CORE.setting:948
  in method new at gen/jvm/CORE.setting:934
  in block <unit> at -e:1

$ perl6-m --version
This is perl6 version 2015.02-151-g669f482 built on MoarVM version 
2015.02-17-g86d0c68