Added for registerElement checking

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 62c902c63a633c628b95b2405f66b4749386dc14..a77cddabed6779e9ed70265014261f84e6af0f3c 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -20428,6 +20428,16 @@ class HtmlDocument extends Document {
 
     // 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.