Add infrastructure to keep track of references to prototypes.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 98d96a8f28ac0747517e1b407753ed869a817ba0..cfd4e62a2339f4aa955f01dceb8d192532d20ad2 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -707,6 +707,7 @@ TYPE_CHECKER(JSContextExtensionObject, JS_CONTEXT_EXTENSION_OBJECT_TYPE)
 TYPE_CHECKER(Map, MAP_TYPE)
 TYPE_CHECKER(FixedArray, FIXED_ARRAY_TYPE)
 TYPE_CHECKER(FixedDoubleArray, FIXED_DOUBLE_ARRAY_TYPE)
+TYPE_CHECKER(WeakFixedArray, FIXED_ARRAY_TYPE)
 TYPE_CHECKER(ConstantPoolArray, CONSTANT_POOL_ARRAY_TYPE)
 
 
@@ -2368,6 +2369,39 @@ void FixedDoubleArray::FillWithHoles(int from, int to) {
 }
 
 
+Object* WeakFixedArray::get(int index) const {
+  Object* raw = FixedArray::cast(this)->get(index + kFirstIndex);
+  if (raw->IsSmi()) return raw;
+  return WeakCell::cast(raw)->value();
+}
+
+
+bool WeakFixedArray::IsEmptySlot(int index) const {
+  DCHECK(index < length());
+  return get(index)->IsSmi();
+}
+
+
+void WeakFixedArray::clear(int index) {
+  FixedArray::cast(this)->set(index + kFirstIndex, Smi::FromInt(0));
+}
+
+
+int WeakFixedArray::length() const {
+  return FixedArray::cast(this)->length() - kFirstIndex;
+}
+
+
+int WeakFixedArray::last_used_index() const {
+  return Smi::cast(FixedArray::cast(this)->get(kLastUsedIndexIndex))->value();
+}
+
+
+void WeakFixedArray::set_last_used_index(int index) {
+  FixedArray::cast(this)->set(kLastUsedIndexIndex, Smi::FromInt(index));
+}
+
+
 void ConstantPoolArray::NumberOfEntries::increment(Type type) {
   DCHECK(type < NUMBER_OF_TYPES);
   element_counts_[type]++;
@@ -3362,6 +3396,7 @@ CAST_ACCESSOR(Struct)
 CAST_ACCESSOR(Symbol)
 CAST_ACCESSOR(UnseededNumberDictionary)
 CAST_ACCESSOR(WeakCell)
+CAST_ACCESSOR(WeakFixedArray)
 CAST_ACCESSOR(WeakHashTable)