Get rid of the unused native code StringAddStub.

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 3592 matching lines @@
   generator.GenerateSlow(masm_, call_helper);
 
   __ bind(&done);
   context()->Plug(result);
 }
 
 
 void FullCodeGenerator::EmitStringAdd(CallRuntime* expr) {
   ZoneList<Expression*>* args = expr->arguments();
   ASSERT_EQ(2, args->length());
+  VisitForStackValue(args->at(0));
+  VisitForAccumulatorValue(args->at(1));
 
-  if (FLAG_new_string_add) {
-    VisitForStackValue(args->at(0));
-    VisitForAccumulatorValue(args->at(1));
-
-    __ pop(rdx);
-    NewStringAddStub stub(STRING_ADD_CHECK_BOTH, NOT_TENURED);
-    __ CallStub(&stub);
-  } else {
-    VisitForStackValue(args->at(0));
-    VisitForStackValue(args->at(1));
-
-    StringAddStub stub(STRING_ADD_CHECK_BOTH);
-    __ CallStub(&stub);
-  }
+  __ pop(rdx);
+  StringAddStub stub(STRING_ADD_CHECK_BOTH, NOT_TENURED);
+  __ CallStub(&stub);
   context()->Plug(rax);
 }
 
 
 void FullCodeGenerator::EmitStringCompare(CallRuntime* expr) {
   ZoneList<Expression*>* args = expr->arguments();
   ASSERT_EQ(2, args->length());
 
   VisitForStackValue(args->at(0));
   VisitForStackValue(args->at(1));
@@ skipping 1262 matching lines @@
 
   ASSERT_EQ(isolate->builtins()->OsrAfterStackCheck()->entry(),
             Assembler::target_address_at(call_target_address));
   return OSR_AFTER_STACK_CHECK;
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64