FULL PRODUCT VERSION :
Java(TM) SE Runtime Environment (build 1.8.0-ea-b119)
Java HotSpot(TM) 64-Bit Server VM (build 25.0-b61, mixed mode)
A DESCRIPTION OF THE PROBLEM :
String.prototype.toLowerCase() [ES5.1 - 18.104.22.168] needs to apply the additional case folding rules from SpecialCasing.txt. This does not or rather no longer work correctly in Nashorn on Java8. This has regressed because of JDK-8020037.
The bug report for JDK-8020037 looks wrong to me, also see JDK-7036910 for a related discussion.
STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
jjs> "\u0130".toLowerCase() === "\u0069\u0307"
This bug can be reproduced always.