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

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index 7a60adc0b37db09bf368fc6ee61556f461d016a6..49adf6af660454adabaf994c0c18bf946e974be7 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -1459,10 +1459,13 @@ void FullCodeGenerator::VisitObjectLiteral(ObjectLiteral* expr) {
       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;