Fix widgets demo.

sky/examples/widgets/index.sky

 <!--
 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 -->
 <sky>
-<import src="/sky/framework/sky-element/sky-element.sky"/>
+<import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement"/>
 <import src="/sky/framework/sky-button/sky-button.sky"/>
 <import src="/sky/framework/sky-box/sky-box.sky"/>
 <import src="/sky/framework/sky-checkbox/sky-checkbox.sky"/>
 <import src="/sky/framework/sky-radio/sky-radio.sky"/>
 <sky-element name="widget-root">
 <template>
   <style>
   div { display: paragraph; }
   </style>
 
@@ skipping 29 matching lines @@
   attached() {
     this.myButton = this.shadowRoot.getElementById('button');
     this.myCheckbox = this.shadowRoot.getElementById('checkbox');
   }
 }.register();
 </script>
 </sky-element>
 
 <widget-root />
 </sky>