如何使用Polymer添加动态扩展元素？

I've got button:

我有按钮:

<link rel="import" href="/go.html?url=../bower_components/polymer/polymer.html">
<link rel="import" href="/go.html?url=../bower_components/core-input/core-input.html">

<polymer-element name="count-button" extends="button" on-click="increment">
  <template>
    <content>Increment: </content>
  </template>

  <script>
    Polymer({
      counter: 0,
      increment: function(e, detail, sender) {
          this.counter++;
          alert(this.counter);
      }
    })
  </script>
</polymer-element>
<link rel="import" h