Stores to external references don't need write barriers.

src/hydrogen-instructions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 5356 matching lines @@
 inline bool ReceiverObjectNeedsWriteBarrier(HValue* object,
                                             HValue* new_space_dominator) {
   if (object->IsInnerAllocatedObject()) {
     return ReceiverObjectNeedsWriteBarrier(
         HInnerAllocatedObject::cast(object)->base_object(),
         new_space_dominator);
   }
   if (object->IsConstant() && HConstant::cast(object)->IsCell()) {
     return false;
   }
+  if (object->IsConstant() &&
+      HConstant::cast(object)->HasExternalReferenceValue()) {
+    // Stores to external references require no write barriers
+    return false;
+  }
   if (object != new_space_dominator) return true;
   if (object->IsAllocate()) {
     return !HAllocate::cast(object)->IsNewSpaceAllocation();
   }
   return true;
 }
 
 
 class HStoreGlobalCell: public HUnaryOperation {
  public:
@@ skipping 761 matching lines @@
   }
   HValue* new_space_dominator() const { return new_space_dominator_; }
 
   bool NeedsWriteBarrier() {
     ASSERT(!(FLAG_track_double_fields && field_representation().IsDouble()) ||
            transition_.is_null());
     if (IsSkipWriteBarrier()) return false;
     if (field_representation().IsDouble()) return false;
     if (field_representation().IsSmi()) return false;
     if (field_representation().IsInteger32()) return false;
+    if (field_representation().IsExternal()) return false;
     return StoringValueNeedsWriteBarrier(value()) &&
         ReceiverObjectNeedsWriteBarrier(object(), new_space_dominator());
   }
 
   bool NeedsWriteBarrierForMap() {
     if (IsSkipWriteBarrier()) return false;
     return ReceiverObjectNeedsWriteBarrier(object(), new_space_dominator());
   }
 
   virtual void FinalizeUniqueValueId() {
@@ skipping 790 matching lines @@
   virtual bool IsDeletable() const { return true; }
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_