title: "Uncaught ReferenceError: this is not defined" in class constructor
body: <p>I am playing with the new stuff in JavaScript/ES6. I get an <code>Uncaught ReferenceError: this is not defined(...) player.js:5</code> in my code. As far as I see, there are no errors here! Is this a bug? Any workarounds?</p>
  
  <p>index.html</p>
  
  <pre><code>&lt;html&gt;
      &lt;head&gt;
          &lt;script type="text/javascript" src="js/entity.js"&gt;&lt;/script&gt;
          &lt;script type="text/javascript" src="js/player.js"&gt;&lt;/script&gt;
          &lt;link href="css/style.css" rel="stylesheet" type="text/css"&gt;
          &lt;title&gt;Test&lt;/title&gt;
      &lt;/head&gt;
      &lt;body&gt;
          &lt;canvas id="screen" width=500 height=500&gt;&lt;/canvas&gt;
          &lt;script type="text/javascript"&gt;initialize();&lt;/script&gt;
      &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>entity.js</p>
  
  <pre><code>"use strict";
  
  class Entity {
      constructor() {
          console.log("Entity");
      }
  }
  </code></pre>
  
  <p>player.js</p>
  
  <pre><code>"use strict";
  
  class Player extends Entity {
      constructor() {
          console.log("Created"); // &lt;- error here
      }
  }
  </code></pre>