Do they not create bootable, installable images of Darwin anymore?
http://www.puredarwin.org/ looks abandoned, and OpenDarwin disappeared long ago - http://www.applematters.com/article/opendarwin-dies-a-lonely...
Ages? https://opensource.apple.com/ looks like it has for a while, except for the addition of 10.10 (I grab CFLite, libclosure, libdispatch, and objc4 whenever they're made available after a release).