They don't work right with the old flexboxes apparently.
text-overflow seems to work (see attached). The problem I was seeing was
that the flex items were display: inline-flex (second box in the test case).
Created attachment 145926 [details]
We need to add some test cases for text-overflow.
display:inline-flex isn't implemented yet (it was, but then rolled out due to a crash). Bug 77772 is for display: inline-flex.
Related bug 94584.