我正在尝试使用l2t-paper-rating但它没有显示。我已经导入了必要的组件。我可以在阴影dom中看到它，但它不可见。下面是代码。
<link rel="import" href="../bower_components/iron-icon/iron-icon.html">
<link rel="import" href="../bower_components/iron-icons/iron-icons.html">
<link rel="import" href="../bower_components/iron-a11y-keys-behavior/iron-a11y-keys-behavior.html">
<link rel="import" href="../bower_components/paper-icon-button/paper-icon-button.html">
<link rel="import" href="../bower_components/l2t-paper-rating/l2t-paper-rating.html">
.......
........
<div>
<l2t-paper-rating rating="4"></l2t-paper-rating>
</div>