gistfile1.html

<link rel="import" href="../bower_components/core-collapse/core-collapse.html">

<polymer-element name="my-collapse">
    <template>
        <button on-click="{{toggle}}">Click me</button>

        <core-collapse id="collapse">
            Hello world!
        </core-collapse>
    </template>
    <script>
        Polymer('my-collapse', {
            toggle: function() {
                this.$.collapse.toggle();
            }
        });
    </script>
</polymer-element>