title: Polymer add class when property is true
body: <p>I have seen this in angular before and wondered if this is possible in polymer as well. <a href="https://stackoverflow.com/questions/7792652/what-is-the-best-way-to-conditionally-apply-a-class">Angular - What is the best way to conditionally apply a class?</a></p>
  
  <p>I have set up a property named 'animated':</p>
  
  <pre><code>animated: {
    type: Boolean,
    value: false,
  },
  </code></pre>
  
  <p>When animated is true, a div inside my element should have a css class of <strong>.animate</strong>.</p>
  
  <pre><code>&lt;div class=""&gt;&lt;/div&gt;
  </code></pre>
  
  <p>For now I have done that inside of the ready function.
  But since I came across that Stackoverflow question I wondered if this is prossible in polymer.</p>
  
  <p>Thanks!</p>