Hi!

   Glad to see someone still pokes at darwinbuild from time to time.

   I was just poking at it the other day too.  Not sure if you found a way to download the sources, but I also ported it to Sierra by fixing the debug stuff and the obsolete methods.  I also submitted my changes back to GitHub and put in a pull request... in case anyone else is interested.

   Anyway, as for downloading the sources, I just grabbed them all from Apple using this command:

darwinxref version '*' | while read f; do curl https://opensource.apple.com/tarballs/`echo $f | sed -e 's/-.*//g'`/$f.tar.gz > Sources/$f.tar.gz; done

   Not super elegant, I'm sure, but gets the job done in one shot.  

   Other challenges I've run into are dealing with newer versions of Xcode, and the fact that Apple hasn't released the sources for the Xcode stuff in quite a while...

Anyway, hope your project is going well!

-Ben