Added for registerElement checking

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 03f4b642a8c365fd6f6fad41b3a9be48b2ab36e4..3f651f2044d146020a1983d503f2caaa8c460abe 100644
--- a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
@@ -357,6 +357,16 @@ $else
 
     // Figure out which DOM class is being extended from the user's Dart class.
     var classMirror = reflectClass(customElementClass);
+
+    var locationUri = classMirror.location.sourceUri.toString();
+    if (locationUri == 'dart:html' || locationUri == 'dart:svg') {
+      throw new DomException.jsInterop("HierarchyRequestError: Cannot register an existing dart:html or dart:svg type.");
+    }
+
+    if (classMirror.isAbstract) {
+      throw new DomException.jsInterop("HierarchyRequestError: Cannot register an abstract class.");
+    }
+
     var jsClassName = _getJSClassName(classMirror);
     if (jsClassName == null) {
       // Only components derived from HTML* can be extended.