to appear on top of other content by setting its `z-index` property. You
  must ensure no element has a stacking context with a higher `z-index` than its parent stacking
  context. You should place this element as a child of the body whenever possible.

  @demo demo/index.html
  @polymerBehavior
  */
  Polymer.IronOverlayBehavior = [Polymer.IronFitBehavior, Polymer.IronResizableBehavior, Polymer.IronOverlayBehaviorImpl];

  /**
   * Fired after the overlay opens.
   * @event iron-overlay-opened
   */

  /**