Allow on-* event handlers on <sky-element>.

sky/framework/sky-button/sky-button.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.
 -->
 <import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement" />
 
-<sky-element name="sky-button" attributes="highlight:boolean">
+<sky-element
+    name="sky-button"
+    attributes="highlight:boolean"
+    on-mousedown="handleMouseDown"
+    on-mouseup="handleMouseUp"
+    on-mouseout="handleMouseOut">
 <template>
   <style>
     :host {
       display: inline-flex;
       border-radius: 4px;
       justify-content: center;
       align-items: center;
       border: 1px solid blue;
       -webkit-user-select: none;
       margin: 5px;
     }
     :host([highlight=true]) {
       background-color: orange;
     }
   </style>
   <content />
 </template>
 <script>
 module.exports = class extends SkyElement {
   created() {
+    super.created();
+
     this.tabIndex = 0; // Make focusable.
-
-    this.addEventListener('mousedown', function() {
-      this.highlight = true;
-    });
-    this.addEventListener('mouseup', function() {
-      this.highlight = false;
-    });
-    this.addEventListener('mouseout', function() {
-      this.highlight = false;
-    });
+  }
+  handleMouseDown() {
+    this.highlight = true;
+  }
+  handleMouseUp() {
+    this.highlight = false;
+  }
+  handleMouseOut() {
+    this.highlight = false;
   }
 }.register();
 </script>
 </sky-element>