A few more trivial write barrier calls.

Change-Id: I4b0f3c4347eed9a2e0a494ae1382065c026d5c40
diff --git a/src/heap.h b/src/heap.h
index 773789b..1c2b7c9 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -139,7 +139,9 @@
 
   // Must be called if a field of an Object in the heap changes, and before any GC safe-point.
   // The call is not needed if NULL is stored in the field.
-  static void WriteBarrier(const Object* object);
+  static void WriteBarrier(const Object* object) {
+    // TODO: re-enable card marking when concurrent collector is active.
+  }
 
  private:
   // Allocates uninitialized storage.