Simplify EventTarget bindings generation (2)

Source/core/events/EventTarget.idl

Index: Source/core/events/EventTarget.idl
diff --git a/Source/core/events/EventTarget.idl b/Source/core/events/EventTarget.idl
index 0cd1b1718f06791cc18bce0b16e8efaa30c59fdf..7febedfcedc843658d302a0918b9c8853da896d5 100644
--- a/Source/core/events/EventTarget.idl
+++ b/Source/core/events/EventTarget.idl
@@ -21,10 +21,10 @@
 [
     Custom=ToV8,
 ] interface EventTarget {
-    void addEventListener(DOMString type,
+    void addEventListener([TreatNullAs=NullString] DOMString type,
                           EventListener listener,
                           optional boolean useCapture);
-    void removeEventListener(DOMString type,
+    void removeEventListener([TreatNullAs=NullString] DOMString type,
                              EventListener listener,
                              optional boolean useCapture);
     [RaisesException] boolean dispatchEvent(Event event);