As per http://dev.w3.org/csswg/css3-flexbox/#flex-baselines, it should be the first row in the table. I think right now we use the last row.
From the spec:
CSS 2.1 did not define the baseline of block or table boxes. It is expected that they will be defined consistent with those of table cells, as follows:
The inline-axis baseline of a block is the baseline of the first in-flow line box in the block, or the first in-flow block-level child in the block that has a baseline, whichever comes first. If there is no such line box or child, then the block has no baseline. For the purposes of finding a baseline, in-flow boxes with a scrolling mechanisms (see the ‘overflow’ property) must be considered as if scrolled to their origin position.
A block has no block-axis baseline.
The inline-axis baseline of a table box is the baseline of its first row. However, when calculating the baseline of an inline-block, table boxes must be skipped.
The block-axis baseline of a table is undefined.