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

sky/tests/resources/test-element.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="test-element" attributes="size:number, checked:boolean,name:string">
+<sky-element
+    name="test-element"
+    attributes="size:number, checked:boolean,name:string"
+    on-host-event="handleEvent">
 <template>
   <div id="inside" on-test-event="handleEvent" lang="{{ value }}">{{ value }}</div>
 </template>
 <script>
 module.exports = class extends SkyElement {
   created() {
     this.lastEvent = null;
     this.value = 10;
     this.shadowRootReadyCount = 0;
     this.changes = [];
@@ skipping 16 matching lines @@
   recordAttributeChange(name, oldValue, newValue) {
     this.changes.push({
       name: name,
       newValue: newValue,
       oldValue: oldValue,
     });
   }
 }.register();
 </script>
 </sky-element>