title: paper-dropdown-menu selected property issue
body: <p><strong><em>Hello:</em></strong></p>
  
  <p>I have a <code>paper-dropdown-menu</code> that is filled from an observable variable, whenever the variable changes I want to re-select the first item. The first time it works correctly however, the next times it keeps selecting the same first element that was first selected. This is the component's html and dart code;</p>
  
  
  
  <h3>paper_dropdown_menu_bug_test.dart</h3>
  
  <pre class="lang-dart prettyprint-override"><code>import 'package:polymer/polymer.dart';
  
  @CustomTag('paper-dropdown-menu-bug-test')
  class PaperDropdownMenuBugTest extends PolymerElement {
    @observable List list;
  
    PaperDropdownMenuBugTest.created() : super.created(){
      this.list = [0, 1, 2];    
    }
  
    listChanged(){
      this.$["paper-dropdown-menu"].selected = 0;
    }
  
    changeList(){
      this.list = [3, 4, 5]; 
    }
  }
  </code></pre>
  
  <h3>paper_dropdown_menu_bug_test.html</h3>
  
  <pre class="lang-dart prettyprint-override"><code>&lt;link rel="import" href="packages/polymer/polymer.html"&gt;
  &lt;link rel="import" href="packages/paper_elements/paper_dropdown_menu.html"&gt;
  &lt;link rel="import" href="packages/paper_elements/paper_item.html"&gt;
  
  &lt;polymer-element name="paper-dropdown-menu-bug-test"&gt;
    &lt;template&gt;
      &lt;paper-dropdown-menu id="paper-dropdown-menu"&gt;
        &lt;template repeat="{{item in list}}"&gt;
          &lt;paper-item label="{{item}}"&gt;&lt;/paper-item&gt;
        &lt;/template&gt;
      &lt;/paper-dropdown-menu&gt;
      &lt;button on-click="{{changeList}}"&gt;Change List&lt;/button&gt;
    &lt;/template&gt;
    &lt;script type="application/dart" src="paper_dropdown_menu_bug_test.dart"&gt;&lt;/script&gt;
  &lt;/polymer-element&gt;
  </code></pre>
  
  <p>Am I Doing it wrong, is there a workaround or should I fill a bug report? (<em>I have already tried selecting another index and then 0, but does not work</em>)</p>
  
  <p><em>Thanks in advance</em></p>