Automate reflected properties in SkyElement.

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">
+<sky-element name="sky-button" attributes="highlight:boolean">
 <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></content>
+  <content />
 </template>
 <script>
 module.exports = class extends SkyElement {
   created() {
     this.tabIndex = 0; // Make focusable.
-    this.setHighlight(false);
 
     this.addEventListener("mousedown", function() {
-      this.setHighlight(true);
+      this.highlight = true;
     });
     this.addEventListener("mouseup", function() {
-      this.setHighlight(false);
+      this.highlight = false;
     });
     this.addEventListener("mouseout", function() {
-      this.setHighlight(false);
+      this.highlight = false;
     });
   }
-  setHighlight(newValue) {
-    // Set both a property and an attribute to keep both parents happy.
-    this.setAttribute('highlight', newValue);
-    this.highlight = newValue;
-  }
 }.register();
 </script>
 </sky-element>