Looks like r229291 contained a half-renamed variable in it, so it just doesn't work.
Created attachment 336379 [details] patch
Comment on attachment 336379 [details] patch r=me
Comment on attachment 336379 [details] patch Clearing flags on attachment: 336379 Committed r229900: <https://trac.webkit.org/changeset/229900>
All reviewed patches have been landed. Closing bug.
<rdar://problem/38797356>
Dan was looking at adding tests for this script.
The title of this bug is misleading. It seems to indicate that the script is wrong, but in reality it just saying that there was a runtime error.