I upgraded to polymer_elements: 1.0.0-rc.6 and all my class='horizontal layout' now are displayed as vertical.
.html
<div class = "layout horizontal">
<span class = "margin-rt-5px">[[prefixx]]</span>
<paper-toggle-button
on-change = "toggleEvent">[[suffixx]]
</paper-toggle-button>
</div>
The div's children are vertically aligned, not horizontally as expected.
When I revert to polymer_elements: 1.0.0-rc.5, the horizontal layout is as expected.
There is some break in the API.
Thanks