[2.0-preview] Dom-if is displayed as a page

[myfrom]

If we include <template is="dom-if> inside iron-swipeable-pages it's considered as a page

Expected outcome

dom-if is ignored and not displayed

Actual outcome

dom-if is displayed as a page

Steps to reproduce

1. Setup conditional iron-swipeable-pages demo using Polymer 2.0 (how to make it work: Fix demos for 2.0-preview #63)
2. Add one conditional dom-if inside iron-swipeable-pages
3. Open it and try to swipe between pages, you'll notice that some pages are blank, those pages are dom-if elements