Stores to external references don't need write barriers.

src/ia32/lithium-codegen-ia32.cc

 // 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 4324 matching lines @@
 }
 
 
 void LCodeGen::DoStoreNamedField(LStoreNamedField* instr) {
   Representation representation = instr->representation();
 
   HObjectAccess access = instr->hydrogen()->access();
   int offset = access.offset();
 
   if (access.IsExternalMemory()) {
+    ASSERT(!instr->hydrogen()->NeedsWriteBarrier());
     MemOperand operand = instr->object()->IsConstantOperand()
         ? MemOperand::StaticVariable(
             ToExternalReference(LConstantOperand::cast(instr->object())))
         : MemOperand(ToRegister(instr->object()), offset);
     if (instr->value()->IsConstantOperand()) {
       LConstantOperand* operand_value = LConstantOperand::cast(instr->value());
       __ mov(operand, Immediate(ToInteger32(operand_value)));
     } else {
       Register value = ToRegister(instr->value());
       __ mov(operand, value);
@@ skipping 2157 matching lines @@
                               FixedArray::kHeaderSize - kPointerSize));
   __ bind(&done);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32