Add two way data binding.

sky/tests/resources/test-element.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.
 -->
 <import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement" />
+<import src="/sky/framework/sky-checkbox/sky-checkbox.sky" />
 
 <sky-element
     name="test-element"
     attributes="size:number, checked:boolean,name:string"
     on-host-event="handleEvent">
 <template>
   <div id="inside" on-test-event="handleEvent" lang="{{ value }}">{{ value }}</div>
+  <sky-checkbox id="checkbox" checked="{{ checked }}" />
 </template>
 <script>
 module.exports = class extends SkyElement {
   created() {
     this.lastEvent = null;
     this.value = 10;
     this.shadowRootReadyCount = 0;
     this.changes = [];
   }
   handleEvent(event) {
@@ skipping 14 matching lines @@
   recordAttributeChange(name, oldValue, newValue) {
     this.changes.push({
       name: name,
       newValue: newValue,
       oldValue: oldValue,
     });
   }
 }.register();
 </script>
 </sky-element>