I know of this compilation of the different iron-flex-layout
classes on Plunker: http://plnkr.co/edit/xIARqr?p=preview
Kasper Peulen, a community member who is active on the Polymer Slack chatroom, was the one who put it together, so all credit goes his way.
As far as I know personally, I don’t know if there is an official demo like this. The documentation page for iron-flex-layout
is currently empty.