console.error when using unknown attributes in templates.

sky/examples/widgets/index.sky

Index: sky/examples/widgets/index.sky
diff --git a/sky/examples/widgets/index.sky b/sky/examples/widgets/index.sky
index 8e74c77c71974ecb4c1e20ba511c73fa92c52198..d4e55c0d1082b62a3a33a4acdf95a78270cd3f37 100644
--- a/sky/examples/widgets/index.sky
+++ b/sky/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');