Add declarataive event handlers.

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" 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>
 
   <sky-box title='Buttons'>
-    <sky-button id='button'>Button</sky-button>
+    <sky-button id='button' on-click='handleClick'>Button</sky-button>
     <div>highlight: {{ myButton.highlight }}</div>
+    <div>clickCount: {{ clickCount }}</div>
   </sky-box>
 
   <sky-box title='Checkboxes'>
     <div><sky-checkbox id='checkbox' />Checkbox</div>
     <div>highlight: {{ myCheckbox.highlight }}</div>
     <div>checked: {{ myCheckbox.checked }}</div>
 
     <div><sky-checkbox id='checkbox' checked='true'/>Checkbox, default checked.</div>
   </sky-box>
 
   <sky-box title='Radios'>
     <sky-box title='Group One'>
       <div><sky-radio group='foo'/>one</div>
       <div><sky-radio group='foo' selected='true' />two</div>
       <div><sky-radio group='foo'/>three</div>
     </sky-box>
     <sky-box title='Group Two'>
       <div><sky-radio group='bar'/>A</div>
       <div><sky-radio group='bar'/>B</div>
       <div><sky-radio group='bar' selected='true' />C</div>
     </sky-box>
   </sky-box>
 
 </template>
 <script>
 module.exports = class extends SkyElement {
   attached() {
     this.myButton = this.shadowRoot.getElementById('button');
     this.myCheckbox = this.shadowRoot.getElementById('checkbox');
+    this.clickCount = 0;
+  }
+  handleClick(event) {
+    this.clickCount++;
   }
 }.register();
 </script>
 </sky-element>
 
 <widget-root />
 </sky>