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
http://openjdk.java.net/guide/index.html
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 nashorn-dev@openjdk.java.net . Responses may be a little slow over the holidays, but we will respond.