[0.8] x-repeat failing to stamp instances on safari

[tjsavage]

I've gone pretty far down the rabbit hole, but I think it's time to pass off to you @kevinpschaaf

To see the weirdness:

• Check out iron-selector's behaviors branch
• Run test/template-repeat.html, I'd just focus on the 'supports repeated items' test.

Notice that:

• After setting the items, the template properly debounces and calls render.
• Inside render, it is properly calling _insertRow -> _generateRow -> stamp
• The result of a stamp is a completely busted row (missing node/fragment, no children)

[notwaldorf]

This is caused by https://github.com/webcomponents/webcomponentsjs/issues/362, and will be fixed when that's fixed.