Fixed to allow only HtmlElement in registerElement

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index a77cddabed6779e9ed70265014261f84e6af0f3c..b56e3741222131c0e1da7e874447e92356b355ff 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -20429,8 +20429,9 @@ class HtmlDocument extends Document {
     // Figure out which DOM class is being extended from the user's Dart class.
     var classMirror = reflectClass(customElementClass);
 
+    var isHtmlElement = classMirror.qualifiedName == #dart.dom.html.HtmlElement;
     var locationUri = classMirror.location.sourceUri.toString();
-    if (locationUri == 'dart:html' || locationUri == 'dart:svg') {
+    if (!isHtmlElement && (locationUri == 'dart:html' || locationUri == 'dart:svg')) {
       throw new DomException.jsInterop("HierarchyRequestError: Cannot register an existing dart:html or dart:svg type.");
     }