console.error when using unknown attributes in templates.

Log an error whenever an element has an unknown attribute in a template.
This means you can't use generic attributes like Polymer, but we
probably want to discourage that anyway since attribute selectors should
be avoided for most things.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/845523004

examples/city-list/city-list.sky

@@ -162,7 +162,7 @@ module.exports.CityItemElement = class extends SkyElement {
   }
 
   </style>
-  <div id="scroller" fit>
+  <div id="scroller">
     <div id="scrollarea">
       <div id="contentarea">
       </div>

examples/widgets/index.sky

@@ -56,6 +56,12 @@
 </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');