<dom-if> support

<dom-if> is now supported:

<iron-lazy-pages
    attr-for-selected="data-route"
    selected="{{route}}"
    loading="{{loading}}"
    hide-immediately>
  <template is="dom-if" data-route="foo" restamp>
    Leaving this tab and coming back will loose input value due to restamp<br/>
    <input type="text"/>
  </template>
  <template is="dom-if" data-route="bar">
    Leaving this tab and coming back will keep input value<br/>
    <input type="text"/>
  </template>
</iron-lazy-pages>