Stores to external references don't need write barriers.

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 4ecb733508ebb544099c352acd67045dc2c32d7f..5a199eaa22ebbe5dd28f453c5a42118b70d145e5 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -2237,15 +2237,15 @@ LInstruction* LChunkBuilder::DoStoreNamedField(HStoreNamedField* instr) {
       instr->NeedsWriteBarrierForMap();
 
   LOperand* obj;
-  if (needs_write_barrier) {
-    obj = is_in_object
-        ? UseRegister(instr->object())
-        : UseTempRegister(instr->object());
-  } else if (is_external_location) {
+  if (is_external_location) {
     ASSERT(!is_in_object);
     ASSERT(!needs_write_barrier);
     ASSERT(!needs_write_barrier_for_map);
     obj = UseRegisterOrConstant(instr->object());
+  } else if (needs_write_barrier) {
+    obj = is_in_object
+        ? UseRegister(instr->object())
+        : UseTempRegister(instr->object());
   } else {
     obj = needs_write_barrier_for_map
         ? UseRegister(instr->object())