my_element.dart

@HtmlImport('my_element_nodart.html')
library my_element;

import 'package:polymer/polymer.dart';

@CustomTag('my-element')
class MyElement extends PolymerElement {
  MyElement.created() : super.created();
}

my_element.html

<!-- Import this file from your HTML files. -->

<link rel="import" href="my_element_nodart.html>
<script type="application/dart" src="my_element.dart"></script>

my_element_nodart.html

<!--
  Import this from your Dart file (same one loaded from my_element.html).
  You may want to stick this inside a `src` directory.
-->

<polymer-element name="my-element">
  <template>
  </template>
</polymer-element>