Chrome Version       : 54.0.2833.0
URL : https://jsbin.com/zumevobeko/edit?html,output 


1. Open JSBin link. Observe that items are stacked on top of one another, even though the iron-list should lay out items vertically

2. Open https://output.jsbin.com/zumevobeko

3. Open devtools, place a breakpoint in https://polygit.org/components/iron-list/iron-list.html:_updateMetrics() (line 1200), refresh

4. When the breakpoint is triggered, switch to the elements pane. Select one of the .row elements and observe that the expected styles are shown in the ‘Styles’ tab.

5. Switch to the ‘Computed’ tab. Note that according to this panel, the item is display:none and has none of the expected styles. There’s no obvious reason for the element to be display: none at this point.

6. In the jsbin, comment out the Polymer.Base.importHref line. Everything now renders as expected.

Note that issue is not in iron-list, but iron-list expects that all of the elements it creates have non-zero height. Stamping zero-height elements causes it to continue stamping in loop for several seconds.