title: Hidden attribute in Polymer 1.0
body: <p>I haven't used Polymer since the 0.4-0.5 era and am used to using the hidden attribute like so <code>&lt;my-element hidden="{{foo != bar}}"&gt;&lt;/my-element&gt;</code></p>
  
  <p>Now in Polymer 1.0 I see that have to use computed values from a method for anything that is not straight up a boolean value.
  I have my code like this:</p>
  
  <p><code>&lt;my-element hidden="{{_computeHidden()}}"&gt;&lt;/my-element&gt;</code></p>
  
  <p>And then in the script section:</p>
  
  <pre><code>Polymer({
      is: 'super-element',
      properties: {...},
      _computeHidden: function(){
           console.log('its being called, mkay');
           return !(foo == bar);
      }
  });
  </code></pre>
  
  <p>Now in the console the message comes up twice after page refresh but when the value of <code>foo</code> changes, the element does not disappear. What am I doing wrong?</p>