fix custom elements that extend tags like "section"

tools/dom/src/dart2js_CustomElementSupport.dart

Index: tools/dom/src/dart2js_CustomElementSupport.dart
diff --git a/tools/dom/src/dart2js_CustomElementSupport.dart b/tools/dom/src/dart2js_CustomElementSupport.dart
index f38bae15be20a58ff8fbd7e61bf74a8e8ed87ce9..665cd5e26e61f0529f0948a28241932b37325d68 100644
--- a/tools/dom/src/dart2js_CustomElementSupport.dart
+++ b/tools/dom/src/dart2js_CustomElementSupport.dart
@@ -113,10 +113,8 @@ void _registerCustomElement(context, document, String tag, Type type,
 
   var options = JS('=Object', '{prototype: #}', proto);
 
-  if (baseClassName != 'HTMLElement') {
-    if (extendsTagName != null) {
-      JS('=Object', '#.extends = #', options, extendsTagName);
-    }
+  if (extendsTagName != null) {
+    JS('=Object', '#.extends = #', options, extendsTagName);
   }
 
   JS('void', '#.register(#, #)', document, tag, options);