title: popstate event handler seems not to work
body: <p>I'm having a problem with a 'popstate' event handler, this is my code:</p>
  
  <pre><code>window.addEventListener("popstate", function (event){
      if (event.state) {
          alert('abc')
      }
  });
  
  // The data object is arbitrary and is passed with the popstate event.
  var dataObject = {
      createdAt: '2011-10-10',
      author: 'donnamoss'
  };
  
  var url = '/posts/new-url';
  history.pushState(dataObject, document.title, url);
  </code></pre>
  
  <p>I expected this code will pop up an alert box when it's executed but, nothing happens. </p>
  
  <p>Is there anything wrong here?</p>
  
  <p>Thank you.</p>