Add declarataive event handlers.

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 88e63ce206d5880cccf9c31911b8368f8f7ed112..d102f85baf950deaeb68e1fe9143126b20cb702f 100644
--- a/sky/examples/widgets/index.sky
+++ b/sky/examples/widgets/index.sky
@@ -16,8 +16,9 @@
   </style>
 
   <sky-box title='Buttons'>
-    <sky-button id='button'>Button</sky-button>
+    <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'>
@@ -47,6 +48,10 @@ module.exports = class extends SkyElement {
   attached() {
     this.myButton = this.shadowRoot.getElementById('button');
     this.myCheckbox = this.shadowRoot.getElementById('checkbox');
+    this.clickCount = 0;
+  }
+  handleClick(event) {
+    this.clickCount++;
   }
 }.register();
 </script>