Fixed to allow only HtmlElement in registerElement

tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate

Index: tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
index 3f651f2044d146020a1983d503f2caaa8c460abe..2871d1c7c8dd6666c28f63f088a5f440d1399c1e 100644
--- a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
@@ -358,8 +358,9 @@ $else
     // 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.");
     }