Add support for trait-based virtual methods on Nodes, and use it for before_remove_attr and after_set_attr.

src/components/script/dom/eventtarget.rs

@@ -9,6 +9,7 @@ use dom::bindings::codegen::EventListenerBinding::EventListener;
 use dom::event::Event;
 use dom::eventdispatcher::dispatch_event;
 use dom::node::NodeTypeId;
+use dom::virtualmethods::VirtualMethods;
 use servo_util::str::DOMString;
 
 use collections::hashmap::HashMap;
@@ -123,3 +124,9 @@ impl Reflectable for EventTarget {
         &mut self.reflector_
     }
 }
+
+impl VirtualMethods for JS<EventTarget> {
+    fn super_type(&self) -> Option<~VirtualMethods:> {
+        None
+    }
+}