I'm surprised that this isn't self-evident. Flossing dislodges stuff from between your teeth. Brushing beforehand means you don't have an opportunity to remove that stuff.
In my opinion, it doesn't really matter which you do first, as long as you rinse well after each step. I brush first - get most of it with the brush and rinse, then get the rest of it with the floss and rinse.
2. Brush without toothpaste.
3. Floss
4. Brush with toothpaste.
So I floss during brushing. One pass with the toothbrush, floss, second pass with toothbrush, rinse.
Flossing feels not that different from following up toothbrushing by rinsing (bad): it's removing a bunch of the amazing good stuff the toothpaste is designed to deposit.