title: Paper-dropdown-menu error in observe.js
body: <p>When trying to replicate the paper-dropdown-menu that works so great on polymers website on my apache2 server I am getting this error. When the dropdown button is pressed. </p>
  
  <pre><code>Exception caught during observer callback: TypeError: Cannot read property 'margin' of undefined
  at core-dropdown-overlay.Polymer.positionTarget (http://localhost/components/core-dropdown/core-dropdown-overlay.html:125:26)
  at core-dropdown-overlay.Polymer.updateTargetDimensions (http://localhost/components/core-overlay/core-overlay.html:445:12)
  at core-dropdown-overlay.Polymer.prepareRenderOpened (http://localhost/components/core-overlay/core-overlay.html:347:14)
  at core-dropdown-overlay.Polymer.openedChanged (http://localhost/components/core-overlay/core-overlay.html:306:12)
  at core-dropdown-overlay.g.invokeMethod (http://localhost/components/polymer/polymer.js:12:13312)
  at core-dropdown-overlay.g.notifyPropertyChanges (http://localhost/components/polymer/polymer.js:12:11598)
  at Object.Observer.report_ (http://localhost/components/platform/platform.js:12:12614)
  at Object.createObject.check_ (http://localhost/components/platform/platform.js:12:18112)
  at c (http://localhost/components/platform/platform.js:12:5465) 
  </code></pre>
  
  <p>HTML</p>
  
  <pre><code>&lt;!doctype html&gt;
  &lt;html&gt;
  
  &lt;head&gt;
      &lt;!-- Style Sheets --&gt;
      &lt;!--  &lt;LINK REL=StyleSheet HREF="css/viewer.css" TITLE="ViewerPage"&gt; --&gt;
  
      &lt;!-- Dependencies --&gt;
  
       &lt;!-- Polymer Dependencies --&gt;
      &lt;script src="components/platform/platform.js"&gt;&lt;/script&gt;
  
      &lt;link rel="import" href="components/paper-dropdown-menu/paper-dropdown-menu.html"&gt;
      &lt;link rel="import" href="components/core-elements/core-elements.html"&gt;
      &lt;link rel="import" href="components/paper-elements/paper-elements.html"&gt;
  
  &lt;/head&gt;
  
  &lt;body&gt;
        &lt;paper-dropdown-menu selected="Financier" valueattr="label"&gt;
        &lt;paper-item label="Croissant"&gt;&lt;/paper-item&gt;
        &lt;paper-item label="Donut"&gt;&lt;/paper-item&gt;
        &lt;paper-item label="Financier"&gt;&lt;/paper-item&gt;
        &lt;paper-item label="Madeleine"&gt;&lt;/paper-item&gt;
      &lt;/paper-dropdown-menu&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>When the button is pressed, no other values can be seen or selected.It simply renders as the Financier in a small box with some of the first paper-item viewable.</p>
  
  <p>I have installed all the above dependencies using bower and the commands found on polymer, from what I understand it is suppose to handle dependency issues for me, so I seem to be at a loss as to why I have the above error.</p>
  
  <p>EDIT:</p>
  
  <p>Upon further research it appears this line of code is breaking in components/core-dropdown/core-dropdown-overlay.html:125:26</p>
  
  <pre><code>var dims = this.dimensions;
  var margin = dims.margin;
  </code></pre>