[css3-flexbox] flex-line-pack without flexing

http://dev.w3.org/csswg/css3-flexbox/#flex-line-pack

"Note: Only multi-line flexboxes ever have free space in the cross axis for
lines to be aligned in, because in asingle-line flexbox the sole line
automatically stretches to fill the space."

Technically, if none of the flexitems have flex, then you can also have free
space in the cross-axis. Should probably adjust this note to take that into
account.