[custom_element] fix warning caused by new dart:html methods

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 fbf80ebff6b276b68431e9ce3d58dcff79c80d65..4fc679f0c2425d91dd8acbf7712d9baa6b19bd7e 100644
--- a/pkg/custom_element/lib/custom_element.dart
+++ b/pkg/custom_element/lib/custom_element.dart
@@ -98,11 +98,15 @@ class CustomElement implements Element {
   void inserted() {}
   @deprecated
   void enteredView() {}
+  @deprecated
+  void attached() {}
 
   /** Invoked when this component is removed from the DOM tree. */
   void removed() {}
   @deprecated
   void leftView() {}
+  @deprecated
+  void detached() {}
 
   /** Invoked when any attribute of the component is modified. */
   void attributeChanged(String name, String oldValue, String newValue) =>