title: iron-list [first,last]VisibleIndex bindings don't work?
body: <p>I am using a polymer iron-list and trying to dynamically respond to scrolling to create an infinite list of information.</p>
  
  <p>I'm finding that I can't get data binding to work on the firstVisibleIndex and lastVisibleIndex properties, nor am I able to get events from those properties when they change.</p>
  
  <p>I'm binding the properties to variables like this:</p>
  
  <pre><code>  &lt;iron-list id="list" items="[[items]]" as="item" first-visible-index="{{fvi}}" last-visible-index="{{lvi}}"&gt;
      &lt;template&gt;
        &lt;div&gt;
          &lt;center&gt;[[item.value]]&lt;/center&gt;
        &lt;/div&gt;
      &lt;/template&gt;
    &lt;/iron-list&gt;
    &lt;p&gt;binding: [[fvi]], [[lvi]]&lt;/p&gt;
  </code></pre>
  
  <p>I am able to scroll around, an in my plnkr example I also have a timer that reads the properties using javascript.  The actively read values are correct.  The bound values are always 0.</p>
  
  <p>I tried using an event to trigger the active read (on-first-visible-index-changed) but it doesn't fire.</p>
  
  <p>Any ideas what it takes to get iron-list to cough up these properties?</p>
  
  <p>Complete plnkr here:   <a href="https://plnkr.co/edit/nY7amL?p=preview" rel="nofollow">https://plnkr.co/edit/nY7amL?p=preview</a></p>