Support string add in crankshaft:

src/x64/code-stubs-x64.h

 // Copyright 2011 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 134 matching lines @@
   void GenerateStringAddCode(MacroAssembler* masm);
   void GenerateCallRuntimeCode(MacroAssembler* masm);
   void GenerateLoadArguments(MacroAssembler* masm);
   void GenerateReturn(MacroAssembler* masm);
   void GenerateUninitializedStub(MacroAssembler* masm);
   void GenerateSmiStub(MacroAssembler* masm);
   void GenerateInt32Stub(MacroAssembler* masm);
   void GenerateHeapNumberStub(MacroAssembler* masm);
   void GenerateOddballStub(MacroAssembler* masm);
   void GenerateStringStub(MacroAssembler* masm);
+  void GenerateBothStringStub(MacroAssembler* masm);
   void GenerateGenericStub(MacroAssembler* masm);
 
   void GenerateHeapResultAllocation(MacroAssembler* masm, Label* alloc_failure);
   void GenerateRegisterArgsPush(MacroAssembler* masm);
   void GenerateTypeTransition(MacroAssembler* masm);
   void GenerateTypeTransitionWithSavedArgs(MacroAssembler* masm);
 
   virtual int GetCodeKind() { return Code::TYPE_RECORDING_BINARY_OP_IC; }
 
   virtual InlineCacheState GetICState() {
@@ skipping 164 matching lines @@
   void Print() {
     PrintF("NumberToStringStub\n");
   }
 #endif
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_CODE_STUBS_X64_H_