Stores to external references don't need write barriers.

src/x64/lithium-codegen-x64.cc

 // Copyright 2013 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 3924 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());
     Register value = ToRegister(instr->value());
     if (instr->object()->IsConstantOperand()) {
       ASSERT(value.is(rax));
       LConstantOperand* object = LConstantOperand::cast(instr->object());
       __ store_rax(ToExternalReference(object));
     } else {
       Register object = ToRegister(instr->object());
       __ movq(MemOperand(object, offset), value);
     }
     return;
@@ skipping 1610 matching lines @@
                                FixedArray::kHeaderSize - kPointerSize));
   __ bind(&done);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64