title: variable of object is undefined in event handler
body: <p>The code snippet says it all.
  I have a class and declare its variable in its constructor. The variable works fine inside the constructor, and when i make a new instance of the class, but the variable appears undefiend in the eventHandler.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>"use strict";
  
  class InputEngine{
  
      __mouseDown(event){
    alert(this.mouse); //doesn't work here
   };
   
   constructor(){
       this.mouse =3;
       window.addEventListener( 'mousedown', this.__mouseDown );
   }
  }
  
  let a = new InputEngine();
  alert(a.mouse); //works here</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
  &lt;body&gt;
  &lt;p&gt;Click your mouse.&lt;/p&gt;
  &lt;/body&gt;
  &lt;/html&gt;</code></pre>
  </div>
  </div>
  </p>