title: "with" keyword in Dart
body: <p>Could somebody please write some formal definition of keyword <strong>with</strong> in Dart?</p>
  
  <p>In official Dart examples I have only found: </p>
  
  <pre><code>class TaskElement extends LIElement with Polymer, Observable {
  </code></pre>
  
  <p>But I still can't understand what is it exactly doing.</p>