title: getElementById in Polymer element
body: <p>How can I use getElementById in a Polymer custom element?</p>
  
  <p>Here is my element:</p>
  
  <pre><code>&lt;link rel="import" href="../bower_components/polymer/polymer.html"&gt;
  &lt;link rel="import" href="../styles/shared-styles.html"&gt;
  
  &lt;dom-module id="bb-calendar"&gt;
  
    &lt;template&gt;
  
      &lt;style is="custom-style" include="shared-styles"&gt;&lt;/style&gt;
  
      &lt;div class="card"&gt;
              &lt;paper-toolbar&gt;
                  &lt;div title&gt;Calendar&lt;/div&gt;
              &lt;/paper-toolbar&gt;
              &lt;div id="hideme"&gt;
                  &lt;div&gt;this should be hidden&lt;/div&gt;
              &lt;/div&gt;
      &lt;/div&gt;
  
    &lt;/template&gt;
  
    &lt;script&gt;
  
      Polymer({
  
        is: 'bb-calendar',
  
        ready: function() {
          document.getElementById("hideme").style.display = 'none';
        }
  
      });
  
    &lt;/script&gt;
  
  &lt;/dom-module&gt;
  </code></pre>
  
  <p>When I run the code I get this error message: <code>Uncaught TypeError: Cannot read property 'style' of null</code></p>
  
  <p>Obviously I'm doing something wrong but I don't know what.</p>