title: Polymer dom-repeat item attributes not updating
body: <p>Given this elements template:</p>
  
  <pre><code>&lt;template is="dom-repeat" items="{{_getCatsToDisplay(requestedCats, catObj)}}"&gt;
    &lt;div class="cat-item" title$="{{_getCatTitle(item)}}"&gt;
      &lt;span&gt;example&lt;/span&gt;
    &lt;/div&gt;
  &lt;/template&gt;
  </code></pre>
  
  <p>I am listening to changes in the element's life cycle on <code>attributeChanged</code> to update the <code>title</code> text for each <code>item</code> in the <code>dom-repeat</code>. </p>
  
  <p>Ultimately the object is changing to have an updated title but the DOM is not reflecting the change unless I completely reinstantiate my custom element all together.</p>
  
  <p>How can I make sure the DOM updates when the object or <code>item</code> in the <code>dom-repeat</code> is updated (in this case, the item's title attribute)?</p>