title: Polymer website tutorial not running | undefined is not a function
body: <p>I'm new to polymer and I started to follow this easy first tutorial on the official website
  <a href="http://www.polymer-project.org/docs/start/tutorial/step-1.html" rel="nofollow">http://www.polymer-project.org/docs/start/tutorial/step-1.html</a></p>
  
  <p>all was very easy but nothing was working. Or better, I have only one problem. When I tap on the top menu ("All or favourite") nothing changes and I get this error on console</p>
  
  <pre><code>Uncaught TypeError: undefined is not a function Element.js:104
  Selected: favorites (index):65
  Uncaught TypeError: undefined is not a function Element.js:104
  Exception caught during observer callback: TypeError: undefined is not a function
      at b.classList (http://127.0.0.1/polymer/components/platform/platform.js:13:20941)
      at Polymer.selectedIndexChanged     (data:text/javascript;base64,CgogIFBvbHltZXIoJ3BhcGVyLXRhYnMnLCB7CiAgICAKICA…cG9uZW50cy9wYXBlci10YWJzL3BhcGVyLXRhYnMuaHRtbC9wYXBlci10YWJzLmpzCg==:45:26)
      at e.invokeMethod (http://127.0.0.1/polymer/components/polymer/polymer.js:12:10242)
      at e.notifyPropertyChanges     (http://127.0.0.1/polymer/components/polymer/polymer.js:12:9459)
      at Object.Observer.report_ (http://127.0.0.1/polymer/components/platform/platform.js:12:12892)
      at Object.createObject.check_ (http://127.0.0.1/polymer/components/platform/platform.js:12:18346)
      at Object.global.Platform.performMicrotaskCheckpoint (http://127.0.0.1/polymer/components/platform/platform.js:12:13792)
      at http://127.0.0.1/polymer/components/platform/platform.js:16:24471
      at Object.c [as callback_] (http://127.0.0.1/polymer/components/platform/platform.js:15:9571)
      at Array.c (http://127.0.0.1/polymer/components/platform/platform.js:12:27047) 
  </code></pre>
  
  <p>I thought I did something wrong so I downloaded the example file but I have downloaded the source of the example and I still get the same errors.
  Any help? =)</p>
  
  <p>Thx everyone!</p>