Wrap Custom Element's content nodes

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/polymer/0.2.2/platform.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/polymer/0.2.2/polymer.js"></script>
<my-list>
    <span>First child</span>
    <a>Second child</a>
</my-list>
<hr/>
Desired rendered DOM:
<ul>
    <li><span>First child</span></li>
    <li><a>Second child</a></li>
</ul>    

<polymer-element name="my-list" noscript>
        <template>
            <ul id="container">
                <li><content select="span"></content></li>
                <li><content select=""></content></li>
            </ul>
            leftovers:
            <content></content>
        </template>
    </polymer-element>