[css3-flexbox] absolutely positioned flex item should not have side effect on space distribution

Absolutely positioned flex item should not have side effect on space
distribution. Since space-* is about distribute spaces evenly,

  A--B--C

with B absolutely positioned, should be rendered as

  A--C

, where B is positioned at the middle of A and C if all
'top'/'bottom'/'right'/'left' are 'auto'。


--
hax