Tightening up custom element registration for dart2js

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index fcfeafc96e23a253ac2faaf08deb0f86d9f2e559..9ceaa5bfb8e4c12321459bca2bf97a6d1cecae88 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -32931,7 +32931,18 @@ void _registerCustomElement(context, document, String tag, Type type,
   if (baseClassName == null) {
     throw new ArgumentError(type);
   }
-  if (baseClassName == 'Element') baseClassName = 'HTMLElement';
+
+  if (extendsTagName == null) {
+    if (baseClassName != 'HTMLElement') {
+      throw new UnsupportedError('Class must provide extendsTag if base '
+          'native class is not HTMLElement');
+    }
+  } else {
+    if (!JS('bool', '(#.createElement(#) instanceof window[#])',
+        document, extendsTagName, baseClassName)) {
+      throw new UnsupportedError('extendsTag does not match base native class');
+    }
+  }
 
   var baseConstructor = JS('=Object', '#[#]', context, baseClassName);