Moving prototype transitions into the header of the transition array.

src/transitions.cc

Index: src/transitions.cc
diff --git a/src/transitions.cc b/src/transitions.cc
index 033f224c173e2de3397461f286ef7c972e5127e5..1e965cda716af594d198ab118a01a72bd4a78da8 100644
--- a/src/transitions.cc
+++ b/src/transitions.cc
@@ -45,6 +45,7 @@ MaybeObject* TransitionArray::Allocate(int number_of_transitions) {
   }
 
   array->set(kElementsTransitionIndex, Smi::FromInt(0));
+  array->set(kPrototypeTransitionsIndex, Smi::FromInt(0));
   return array;
 }
 
@@ -98,6 +99,10 @@ MaybeObject* TransitionArray::CopyInsert(String* name, Object* value) {
     result->set_elements_transition(elements_transition());
   }
 
+  if (HasPrototypeTransitions()) {
+    result->SetPrototypeTransitions(GetPrototypeTransitions());
+  }
+
   FixedArray::WhitenessWitness witness(result);
 
   if (insertion_index != kNotFound) {