core-dropdown-base` is a base class for implementing controls that displays
an overlay when tapped on.

The child element with the class `dropdown` will be used as the drop-down. It
should be a `core-dropdown` or other overlay element.

@group Polymer Core Elements
@element core-dropdown-base
@status unstable
@homepage github.io
-->

<link href="../polymer/polymer.html" rel="import">

<polymer-element name="core-dropdown-base" tabindex="0">
<script>

  Polymer({

    publish: {

      /**
       * True if t