Don't set expression attributes before they're bound.

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-box/sky-box.sky"/>
 <import src="/sky/framework/sky-button/sky-button.sky"/>
 <import src="/sky/framework/sky-checkbox/sky-checkbox.sky"/>
 <import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement"/>
@@ skipping 19 matching lines @@
     <div>value = {{ inputValue }}</div>
   </sky-box>
 
   <sky-box title='Buttons'>
     <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><sky-checkbox id='checkbox' checked='{{ checked }}'/>Checkbox</div>
     <div class="output">highlight: {{ myCheckbox.highlight }}</div>
     <div class="output">checked: {{ myCheckbox.checked }}</div>
-    <div><sky-checkbox id='checkbox' checked='{{ checked }}'/>Checkbox, default checked.</div>
+    <div><sky-checkbox id='checkbox' checked="true"/>Checkbox, default checked.</div>
     <div class="output">checked: {{ 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'>
@@ skipping 24 matching lines @@
     this.clickCount++;
     this.checked = !this.checked;
     this.inputValue = "Moar clicking " + this.clickCount;
   }
 }.register();
 </script>
 </sky-element>
 
 <widget-root />
 </sky>