Modify custom elements to inherit from HtmlElement

tools/dom/src/native_DOMImplementation.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of html;
 
 class _Utils {
   static double dateTimeToDouble(DateTime dateTime) =>
       dateTime.millisecondsSinceEpoch.toDouble();
   static DateTime doubleToDateTime(double dateTime) {
@@ skipping 193 matching lines @@
     LibraryMirror lib = cls.owner;
     String libName = lib.uri.toString();
     if (libName.startsWith('dart:')) {
       throw new UnsupportedError("Invalid custom element from $libName.");
     }
     ClassMirror superClass = cls.superclass;
 
     Symbol objectName = reflectClass(Object).qualifiedName;
     bool isRoot(ClassMirror cls) =>
       cls == null || cls.qualifiedName == objectName;
-    Symbol elementName = reflectClass(Element).qualifiedName;
+    // TODO(vsm): Support extending SvgElement as well.
+    Symbol elementName = reflectClass(HtmlElement).qualifiedName;
     bool isElement(ClassMirror cls) =>
       cls != null && cls.qualifiedName == elementName;
 
     while(!isRoot(superClass) && !isElement(superClass)) {
       superClass = superClass.superclass;
     }
 
     if (isRoot(superClass)) {
-      throw new UnsupportedError("Invalid custom element doesn't inherit from Element.");
+      throw new UnsupportedError("Invalid custom element doesn't inherit from HtmlElement.");
     }
     _register(tag, type);
   }
 
   static void _register(String tag, Type type) native "Utils_register";
 }
 
 class _NPObject extends NativeFieldWrapperClass1 {
   _NPObject.internal();
   static _NPObject retrieve(String key) native "NPObject_retrieve";
@@ skipping 186 matching lines @@
   _send(msg) {
     _sendToHelperIsolate(msg, _sendPort);
   }
 
   bool get isActive => _isActive;
 }
 
 get _pureIsolateTimerFactoryClosure =>
     ((int milliSeconds, void callback(Timer time), bool repeating) =>
         new _PureIsolateTimer(milliSeconds, callback, repeating));