title: Conditionally add css class in polymer
body: <p>I have an element inside a polymer component and want to add a css class conditionally.</p>
  
  <pre><code>&lt;div class="bottom {{completed: item.completed}}"&gt;
  </code></pre>
  
  <p>if item.isCompleted is true, then add the .completed class. </p>
  
  <p>However, I've got the following error:</p>
  
  <p>Invalid expression syntax: completed?: item.completed </p>
  
  <p>I don't want to put the hole subtree inside a template conditional. Is it possible to do using an expression inside an html tag? I'm using the last polymer release, is this funtionallity migrated or replaced in any way?</p>