Child elements should support `visibility: collapse`

[doctyper]

Per: http://www.w3.org/TR/css3-flexbox/#cross-sizing

Collapse ‘visibility:collapse’ items. If any flex items have ‘visibility: collapse’, note the cross size of the line they're in as the item's strut size, and restart layout from the beginning.
In this second layout round, when collecting items into lines, treat the collapsed items as having zero main size. For the rest of the algorithm following that step, ignore the collapsed items entirely (as if they were ‘display:none’) except that after calculating the cross size of the lines, if any line's cross size is less than the largest strut size among all the collapsed items in the line, set its cross size to that strut size.

Skip this step in the second layout round.