title: CSS binding with $index when setting multiple classes
body: <p>With this binding</p>
  
  <pre><code>css: 'item' + $index()
  </code></pre>
  
  <p>I get this output</p>
  
  <pre><code>class="item0"
  class="item1"
  class="item2"
  ...
  </code></pre>
  
  <p>I'm not able to use this when binding multiple classes like below, a syntax error is thrown because <code>:</code> is expected instead of <code>+</code>. Any hints?</p>
  
  <pre><code>css: { 'item' + $index(), 'active': $index() == 0 }
  </code></pre>
  
  <p>A workaround is to use attribute binding, but if possible I'd like to avoid that because there are existing classes set already, so I'd have to include those in the binding.</p>
  
  <pre><code>attr: { 'class': 'slider-item item' + $index() + ($index() == 0 ? ' active' : '') }
  </code></pre>