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

sky/framework/sky-checkbox/sky-checkbox.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-button/sky-button.sky" as="SkyButton" />
 
-<sky-element name="sky-checkbox" attributes="checked:boolean">
+<sky-element
+    name="sky-checkbox"
+    attributes="checked:boolean"
+    on-mouseup="handleMouseUp">
 <template>
   <style>
     :host {
       display: flex;
       justify-content: center;
       align-items: center;
       -webkit-user-select: none;
       cursor: pointer;
       width: 30px;
       height: 30px;
@@ skipping 29 matching lines @@
     <div id="check" class="{{ checkStyle }}" />
   </div>
 </template>
 <script>
 module.exports = class extends SkyButton {
   created() {
     super.created();
 
     this.containerStyle = '';
     this.checkStyle = '';
-
-    this.addEventListener('mouseup', function() {
-      this.checked = !this.checked;
-    });
   }
   checkedChanged(oldValue, newValue) {
     this.checkStyle = newValue ? 'checked' : '';
   }
   highlightChanged(oldValue, newValue) {
     this.containerStyle = newValue ? 'highlight' : '';
   }
+  handleMouseUp() {
+    this.checked = !this.checked;
+  }
 }.register();
 </script>
 </sky-element>