After a lot of blood, sweat and tears, we finally got Nashorn into OpenJDK.
hg fclone http://hg.openjdk.java.net/nashorn/jdk8 nashorn~jdk8
or for just Nashorn
hg clone http://hg.openjdk.java.net/nashorn/jdk8/nashorn nashorn
More details about the tools you'll need and using OpenJDK are at
This is just the first drop and needs some larger integration work, but you can browse the source, build and play. Be sure to read the README and RELEASE_README, If you have any questions or issues please send them along to firstname.lastname@example.org . Responses may be a little slow over the holidays, but we will respond.