[perl #131384] .open(:enc) on file with unicode leads to MoarVM panic: MVM_nfg_get_synthetic_info called with out-of-range synthetic

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# New Ticket Created by  Zoffix Znet 
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<Zoffix___> c: HEAD with "/tmp/foo2121".IO { .spurt: "fo♥o"; with 
.open(:enc<ascii>) { say .slurp } }
<committable6> Zoffix___, ¦HEAD(0c5fe56): «MoarVM panic: 
MVM_nfg_get_synthetic_info called with out-of-range synthetic «exit code = 1»»

This is not the result of encoding refactor; still exists in 2017.05:
<Zoffix___> c: 2017.05 with "/tmp/foo2121".IO { .spurt: "fo♥o"; with 
.open(:enc<ascii>) { say .slurp } }
<committable6> Zoffix___, ¦2017.05: «MoarVM panic: MVM_nfg_get_synthetic_info 
called with out-of-range synthetic «exit code = 1»»