See rARC3ffed59. I think that test failure is some sort of weird parser generator bug in Bison 3.0.2 that appears to affect Ubuntu and OS X, at a minimum.
I don't immediately see what the acutal issue is, nor can I find anything suspicious in the changelog. However, I can reproduce the issue reliably locally (OS X).
If I do this:
$ git checkout master $ git clean -df $ ./scripts/build_xhpast.php
...I get test failures in arc.
If I do this:
$ git checkout bison304 # This change $ git clean -df $ ./scripts/build_xhpast.php
...I get clean tests.
The actual AST/parser look fine to me from careful inspection, too.