Make it possible for sky-element to register any tag name

sky/framework/sky-element.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.
 -->
 <script>
 import "dart:mirrors";
 import "dart:sky";
 
 class _Registration {
@@ skipping 16 matching lines @@
 abstract class SkyElement extends Element {
   // Override these functions to receive lifecycle notifications.
   void created() {}
   void attached() {}
   void detached() {}
   void attributeChanged(String attrName, String oldValue, String newValue) {}
   void shadowRootReady() {}
 
   String get tagName => _getTagName(runtimeType);
 
-  // TODO(abarth): Rather than hard-coding "example" here, we should make the
-  // bindings read the |tagName| property of this object during construction.
-  SkyElement() : super("example") {
+  SkyElement() {
     created();
 
     // Invoke attributeChanged callback when element is first created too.
     for (Attr attribute in getAttributes())
       attributeChangedCallback(attribute.name, null, attribute.value);
   }
 
   attachedCallback() {
     if (shadowRoot == null) {
       var registration = _registery[tagName];
@@ skipping 26 matching lines @@
   if (!mirror.isSubclassOf(reflectClass(SkyElement)))
     throw new UnsupportedError('@Tagname can only be used on descendants of SkyElement');
 
   String tagName = _getTagName(type);
   Element template = definition.querySelector('template');
 
   document.registerElement(tagName, type);
   _registery[tagName] = new _Registration(template);
 }
 </script>