Polymer 1.0纸卡未显示

Question

我是HTML的新手，我正试图让这张纸卡显示，但每次我打开它都没有显示的页面。我搞砸了哪里？          

<link rel="import" href="../paper-styles/color.html">
<link rel="import" href="../paper-styles/typography.html">
<link rel="import" href="paper-card.html">

<dom-module id="foo">
  <template>
    <style>
      :host {
        display: block;
      }
    </style>
  </template>

  <paper-card heading="bar"
    <div class="card-content">CardStuff</div>
    <div class="card-actions">
      <paper-button>stuff</paper-button>
    </div>
  </paper-card>

  <script>
    (() => {
      Polymer({
        is: 'foo',
      });
    })();
  </script>
</dom-module>