fix analysis error in custom_element

pkg/custom_element/lib/custom_element.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Custom Elements let authors define their own elements. Authors associate code
  * with custom tag names, and then use those custom tag names as they would any
  * standard tag. See <www.polymer-project.org/platform/custom-elements.html>
  * for more information.
  */
@@ skipping 208 matching lines @@
   get model => host.model;
 
   void set model(newModel) {
     host.model = newModel;
   }
 
   get templateInstance => host.templateInstance;
   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);
   void bind(String name, model, String path) => host.bind(name, model, path);
   void unbind(String name) => host.unbind(name);
   void unbindAll() => host.unbindAll();
   get bindings => host.bindings;
   BindingDelegate get bindingDelegate => host.bindingDelegate;
   set bindingDelegate(BindingDelegate value) { host.bindingDelegate = value; }
 
@@ skipping 456 matching lines @@
       for (var removed in record.removedNodes) {
         if (identical(node, removed)) {
           observer.disconnect();
           element.removed();
           return;
         }
       }
     }
   }).observe(element.parentNode, childList: true);
 }