title: Showing a gray backdrop with a mixin
body: <p>I have a small window that I'm inserting into my page (and removing when the page is closed). I want a grayed-out background behind this window, as on dialogs. So I thought I'd use <code>paper-dialog-behavior</code> or <code>iron-overlay-behavior</code> as a mixin, and set <code>this.withBackdrop = true</code> in my <code>ready()</code> method. However, when I add <code>...extends Polymer.mixinBehaviors([Polymer.IronOverlayBehavior], Polymer.Element)</code> or <code>...extends Polymer.mixinBehaviors([Polymer.PaperDialogBehaviorImpl], Polymer.Element)</code> to this element, it never appears.</p>
  
  <p>I tired <code>.open()</code> like on a dialog and was told that it's undefined. I can trace my element loading in DevTools, and no errors print in the console, but it never appears on the screen.</p>
  
  <p>You can see what I'm going for at this pen: <a href="https://codepen.io/johnthad/pen/zRLMpe" rel="nofollow noreferrer">https://codepen.io/johnthad/pen/zRLMpe</a>
  If I swap the class declarations of <code>MyChild</code> for the one with the mixin, the child loads but never displays.</p>