title: Polymer template repeat not working
body: <p>I've created a custom element and setting the <code>repeat</code> attribute on the root <code>template</code> doesn't seem to have any effect.</p>
  
  <pre><code>&lt;link rel="import" href="/components/polymer/polymer.html"&gt;
  &lt;polymer-element name="cr-tracks"&gt;
      &lt;template repeat="{{track in tracks}}"&gt;
          {{foo}}
          {{track.title}}
      &lt;/template&gt;
  
      &lt;style&gt;
  
      &lt;/style&gt;
  
      &lt;script&gt;
          Polymer('cr-tracks', {
              ready: function() {
                  this.tracks = [
                      {
                          title: "Example Title",
                          artist: "Example Artist"
                      }
                  ];
              },
              foo: 'bar'
          })
      &lt;/script&gt;
  &lt;/polymer-element&gt;
  </code></pre>
  
  <p><code>bar</code>, the value of <code>foo</code> is displayed, but nothing is rendered for <code>track.title</code>. I can access it directly via <code>tracks[0].title</code>.</p>