I would create an <example-ui> so HTML doesn't appear until all widgets are loaded and running.  e.g.,

<body>
  <example-ui></example-ui>
</body>
...

And create an example-ui widget for this example:

example_ui.html
============
<link rel="import" href="packages/spark_widgets/spark-button/spark-button.html">
<polymer-element name="example-ui">
<template>
  <spark-button id="newFile" primary>New File</spark-button>
  <spark-button id="saveFile">Save File</spark-button>
  <spark-button id="deleteFile" active>delete File</spark-button>
</template>
<script type="application/dart" src="example-ui.dart"></script>
</polymer-element>

example_ui.dart
============
library ui;

import 'dart:html';
import 'package:polymer/polymer.dart';

@CustomTag('example-ui')
class ExampleUi extends PolymerElement {
  factory ExampleUi() => new Element.tag('ExampleUi');

  ExampleUi.created() : super.created();
}