[es6] Better support for built-ins subclassing.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 75b7ec1fa698f824e2e82c57da0cb9947132b08f..136c4d600495436ee53608d9ec077d2f0511d456 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -3435,7 +3435,7 @@ void Heap::InitializeJSObjectFromMap(JSObject* obj, FixedArray* properties,
   if (constructor->IsJSFunction() &&
       JSFunction::cast(constructor)->IsInobjectSlackTrackingInProgress()) {
     // We might want to shrink the object later.
-    DCHECK(obj->GetInternalFieldCount() == 0);
+    DCHECK_EQ(0, obj->GetInternalFieldCount());
     filler = Heap::one_pointer_filler_map();
   } else {
     filler = Heap::undefined_value();