[css3-flexbox] Layout algorithm finished, review requested

I've just now finished the "Layout Algorithm" section of the flexbox
spec <http://dev.w3.org/csswg/css3-flexbox/#layout-algorithm>.  I'd
appreciate a review, particularly step 2 of the main algorithm, and
the whole "resolve the flexible lengths" sub-algorithm.  The latter is
based closely off of my JS implementation, which I'm pretty sure gives
ideal results, but I'm not certain (as evidenced by the issue I logged
against it).

~TJ