Get the target field index when transitioning.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index b434a91b16cf816b7bd1746c366580cae0e5ad36..ebcbbd23fd00e259a7305deb9c959670ad66ca82 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1444,6 +1444,13 @@ bool JSObject::TryTransitionToField(Handle<JSObject> object,
 }
 
 
+int JSObject::LastAddedFieldIndex() {
+  Map* map = this->map();
+  int last_added = map->LastAdded();
+  return map->instance_descriptors()->GetFieldIndex(last_added);
+}
+
+
 ACCESSORS(Oddball, to_string, String, kToStringOffset)
 ACCESSORS(Oddball, to_number, Object, kToNumberOffset)