The check for internalized strings relied on the fact that we had less

src/x64/macro-assembler-x64.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 740 matching lines @@
       Label::Distance near_jump = Label::kFar);
 
   void JumpIfBothInstanceTypesAreNotSequentialAscii(
       Register first_object_instance_type,
       Register second_object_instance_type,
       Register scratch1,
       Register scratch2,
       Label* on_fail,
       Label::Distance near_jump = Label::kFar);
 
+  // Checks if the given register or operand is a unique name
+  void JumpIfNotUniqueName(Register reg, Label* not_unique_name,
+                           Label::Distance distance = Label::kFar);
+  void JumpIfNotUniqueName(Operand operand, Label* not_unique_name,
+                           Label::Distance distance = Label::kFar);
+
   // ---------------------------------------------------------------------------
   // Macro instructions.
 
   // Load a register with a long value as efficiently as possible.
   void Set(Register dst, int64_t x);
   void Set(const Operand& dst, int64_t x);
 
   // Move if the registers are not identical.
   void Move(Register target, Register source);
 
@@ skipping 758 matching lines @@
     masm->popfq();                                                           \
   }                                                                          \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_