Throw exception for null listener in addEventListener

sky/engine/bindings/dart_event_listener.h

Index: sky/engine/bindings/dart_event_listener.h
diff --git a/sky/engine/bindings/dart_event_listener.h b/sky/engine/bindings/dart_event_listener.h
index d8a901731b04ebdb39d2467b9c9f03809276eb26..cb293156683250ee3e31e48c5e88eca2cd931662 100644
--- a/sky/engine/bindings/dart_event_listener.h
+++ b/sky/engine/bindings/dart_event_listener.h
@@ -51,12 +51,7 @@ struct DartConverter<EventListener*> {
   static PassRefPtr<EventListener> FromArgumentsWithNullCheck(
       Dart_NativeArguments args,
       int index,
-      Dart_Handle& exception) {
-    Dart_Handle handle = Dart_GetNativeArgument(args, index);
-    if (Dart_IsNull(handle))
-      return nullptr;
-    return FromDart(handle);
-  }
+      Dart_Handle& exception);
 };
 
 }  // namespace blink