Make the runtime entry for setting/changing accessors "atomic".

src/x64/full-codegen-x64.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 1441 matching lines @@
           __ Push(Smi::FromInt(NONE));    // PropertyAttributes
           __ CallRuntime(Runtime::kSetProperty, 4);
         } else {
           __ Drop(3);
         }
         break;
       case ObjectLiteral::Property::SETTER:
       case ObjectLiteral::Property::GETTER:
         __ push(Operand(rsp, 0));  // Duplicate receiver.
         VisitForStackValue(key);
-        __ Push(property->kind() == ObjectLiteral::Property::SETTER ?
-                Smi::FromInt(1) :
-                Smi::FromInt(0));
-        VisitForStackValue(value);
+        if (property->kind() == ObjectLiteral::Property::GETTER) {
+          VisitForStackValue(value);
+          __ PushRoot(Heap::kNullValueRootIndex);
+        } else {
+          __ PushRoot(Heap::kNullValueRootIndex);
+          VisitForStackValue(value);
+        }
         __ Push(Smi::FromInt(NONE));
         __ CallRuntime(Runtime::kDefineOrRedefineAccessorProperty, 5);
         break;
     }
   }
 
   if (expr->has_function()) {
     ASSERT(result_saved);
     __ push(Operand(rsp, 0));
     __ CallRuntime(Runtime::kToFastProperties, 1);
@@ skipping 2887 matching lines @@
   *context_length = 0;
   return previous_;
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64