Fix registerElement for HTMLElement prototype should not change it's m_type

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 7768aad24b928fdf6394d5bb2be7e647ac7ec78b..a52e369a14c8682d4c39a5de0842d88ba9ee5a9b 100644
--- a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
@@ -362,6 +362,10 @@ $if DART2JS
     _registerCustomElement(JS('', 'window'), this, tag, customElementClass,
         extendsTag);
 $else
+    // Hack to setup an interceptor for HTMLElement so it isn't changed when a custom element is created.
+    var jsHTMLElementPrototype = js.JsNative.getProperty(js.JsNative.getProperty(js.context, 'HTMLElement'),'prototype');
+    _blink.Blink_Utils.defineInterceptor(jsHTMLElementPrototype, HtmlElement.instanceRuntimeType);
+
     // Figure out which DOM class is being extended from the user's Dart class.
     var classMirror = reflectClass(customElementClass);