console.error when using unknown attributes in templates.

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 38 matching lines @@
     <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 {
+  created() {
+    this.myButton = null;
+    this.myCheckbox = null;
+    this.myText = null;
+    this.clickCount = 0;
+  }
   attached() {
     this.myButton = this.shadowRoot.getElementById('button');
     this.myCheckbox = this.shadowRoot.getElementById('checkbox');
     this.myText = this.shadowRoot.getElementById('text');
     this.clickCount = 0;
   }
   handleClick(event) {
     this.clickCount++;
     this.myText.value = "Moar clicking " + this.clickCount;
   }
 }.register();
 </script>
 </sky-element>
 
 <widget-root />
 </sky>