third_party/polymer/components/paper-menu-button/metadata.html

<x-meta id="paper-menu-button" label="Menu Button" isContainer>
  <template>
    <paper-menu-button icon="menu">
      <div>Menu Item 1</div>
      <div>Menu Item 2</div>
      <div>Menu Item 3</div>
    </paper-menu-button>
  </template>
  <template id="imports">
    <link rel="import" href="paper-menu-button.html">
  </template>
</x-meta>