diff --git a/src/heap.h b/src/heap.h
index c25803f..aa9db6e 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -40,6 +40,7 @@
   static const size_t kMaximumSize = 16 * MB;
 
   typedef void (RootVisitor)(const Object* root, void* arg);
+  typedef bool (IsMarkedTester)(const Object* object, void* arg);
 
   // Create a heap with the requested sizes. The possible empty
   // image_file_names names specify Spaces to load based on