title: Scope error in constructor (ES6)
body: <p>I've encountered a strange error
  while I was working with the new version of ES6. When I run this piece of code I'm getting <code>ReferenceError: alertBox is not defined</code>. Is there any way to call alertBox inside this function? Thanks in advance :)</p>
  
  <p>Here's the code</p>
  
  <pre><code>class main {
      constructor(data){
      this.data=data;
  
      // this one works
      this.alertBox(this.data);
  
      this.watchFile(function(){
        // this one throws error
        this.alertBox(this.data);
      });
      }
  
    alertBox(data){
      alert(data);
    }
  
    watchFile(cb){
      cb("changed");
    }
  }
  
  // app.js
  new main("hello");
  </code></pre>
  
  <p>Here you can find the snippet: <a href="https://repl.it/FJUo" rel="nofollow noreferrer">https://repl.it/FJUo</a></p>