Fixed custom elements to work in Polymer

tools/dom/templates/html/impl/impl_Document.darttemplate

Index: tools/dom/templates/html/impl/impl_Document.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Document.darttemplate b/tools/dom/templates/html/impl/impl_Document.darttemplate
index fb6d6b1889059268f5cada3b37301a8fb03ecddd..99b2dc02885b1f9edb8710b7bec5bd7c14c048e0 100644
--- a/tools/dom/templates/html/impl/impl_Document.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Document.darttemplate
@@ -83,6 +83,8 @@ $else
       wrapped = wrap_jso(newElement);
       if (wrapped == null) {
         wrapped = wrap_jso_custom_element(newElement);
+      } else {
+        wrapped.blink_jsObject['dart_class'] = wrapped;
       }
     }
 
@@ -126,6 +128,8 @@ $else
       wrapped = wrap_jso(newElement);
       if (wrapped == null) {
         wrapped = wrap_jso_custom_element(newElement);
+      } else {
+        wrapped.blink_jsObject['dart_class'] = wrapped;
       }
     }