fix analysis error in custom_element

pkg/custom_element/lib/custom_element.dart

Index: pkg/custom_element/lib/custom_element.dart
diff --git a/pkg/custom_element/lib/custom_element.dart b/pkg/custom_element/lib/custom_element.dart
index e5c905824cf9f6ce69edb4085af14f36dc4a55b7..369501c69b8ea2ffcca0a75c727b94e9883b70b6 100644
--- a/pkg/custom_element/lib/custom_element.dart
+++ b/pkg/custom_element/lib/custom_element.dart
@@ -226,7 +226,7 @@ class CustomElement implements Element {
   get isTemplate => host.isTemplate;
   get ref => host.ref;
   get content => host.content;
-  DocumentFragment createInstance(model, BindingDelegate delegate) =>
+  DocumentFragment createInstance(model, [BindingDelegate delegate]) =>
       host.createInstance(model, delegate);
   createBinding(String name, model, String path) =>
       host.createBinding(name, model, path);