t to submit the form contents to the server.

  Example:

    <iron-form>
      <form method="get" action="/form/handler">
        <input type="text" name="name" value="Batman">
        <input type="checkbox" name="donuts" checked> I like donuts<br>
        <paper-checkbox name="cheese" value="yes" checked></paper-checkbox>
      </form>
    </iron-form>

By default, a native `<button>` element will submit this form. However, if you
want to submit it from a custom element's click handler, you need to explic