Dart 暂时没有注册元素，但仍在等待注册1个元素

我在用聚合物试飞镖。我想构建一个简单的自定义元素。我基于dart编辑器中聚合物示例应用程序中提供的自定义元素

不幸的是，我得到了以下错误：

No elements registered in a while, but still waiting on 1 elements to be registered. Check that you have a class with an @CustomTag annotation for each of the following tags: 'send-card'

“发送卡”元素的代码如下所示：

send_card.html

<!-- send card -->


<!-- import polymer-element's definition -->
<link rel="import" href="../../packages/polymer/polymer.html">

<link rel="import" href="../../packages/paper_elements/paper_input.html">
<link rel="import" href="../../packages/paper_elements/paper_fab.html">

<link rel="import" href="../../packages/core_elements/core_icons.html">

<polymer-element name="send-card">
  <template>
    <style>
      #input-wrapper {
        width: 500px;
        padding: 5px;
        margin: 10px;
      }
      paper-input {
        color:#5677fc;
      }
      paper-fab {
        background: #5677fc;
        position: relative;
        right: -5px;
        bottom: -5px;
      }
    </style>

    <div id="input-wrapper">
      <paper-input multiline label="Your message"></paper-input>
      <paper-fab icon="add" on-click="{{sendMessage}}"></paper-fab>
    </div>

  </template>
</polymer-element>

在我看来，这里一切都井然有序。有人能解释那个错误吗

---

PS：当我运行代码时，浏览器中不会显示任何内容。

在模板中包含要发送的脚本元素。\u card.dart:

send_card.html：

---

上次我忘记将入口页面添加到聚合物变压器配置时，我就有了这个

另见

import 'package:paper_elements/paper_input.dart';
import 'package:paper_elements/paper_fab.dart';
import 'package:polymer/polymer.dart';

@CustomTag('send-card')
class SendCard extends PolymerElement {

  /// Constructor used to create instance of MainApp.
  SendCard.created() : super.created();

  void sendMessage(){}
}

  <!-- ... -->
  </template>
  <script type="application/dart" src="send_card.dart"></script>
</polymer-element>