Extend the sky color picker example

sky/examples/color/color-wheel.sky

 <!--
 // Copyright 2015 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="color-wheel"
     attributes="color:string"
-    on-pointerdown="handlePointerDown"
-    on-pointermove="handlePointerMove">
+    on-pointerdown="handlePointerDown">
 <template>
   <style>
     img {
       min-width: 100%;
       height: auto;
     }
   </style>
   <img class="color-wheel-img" src="color-wheel.png">
 </template>
 <script>
@@ skipping 36 matching lines @@
 }
 
 function rgbToString(rgb) {
   return "#" + toHexString(rgb.r) + toHexString(rgb.g) + toHexString(rgb.b);
 }
 
 module.exports = class extends SkyElement {
   created() {
     super.created();
     this.color = "#xFF00FF";
+    this.colorChanged = function(){};
   }
   updateColor(event) {
     var bounds = event.target.getBoundingClientRect();
     var x = event.x - bounds.left;
     var y = event.y - bounds.top;
     var radius = Math.min(bounds.width, bounds.height) / 2.0;
     var rgb = xyToRgb(x, y, radius);
     if (rgb)
       this.color = rgbToString(rgb);
   }
   handlePointerDown(event) {
     this.updateColor(event);
   }
-  handlePointerMove(event) {
-    this.updateColor(event);
-  }
 }.register();
 </script>
 </sky-element>