Support string add in crankshaft:

src/ic.h

 // Copyright 2006-2009 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 568 matching lines @@
 // Type Recording BinaryOpIC, that records the types of the inputs and outputs.
 class TRBinaryOpIC: public IC {
  public:
 
   enum TypeInfo {
     UNINITIALIZED,
     SMI,
     INT32,
     HEAP_NUMBER,
     ODDBALL,
+    BOTH_STRING,  // Only used for addition operation.
     STRING,  // Only used for addition operation.  At least one string operand.
     GENERIC
   };
 
   explicit TRBinaryOpIC(Isolate* isolate) : IC(NO_EXTRA_FRAME, isolate) { }
 
   void patch(Code* code);
 
   static const char* GetName(TypeInfo type_info);
 
@@ skipping 42 matching lines @@
 
   Token::Value op_;
 };
 
 // Helper for TRBinaryOpIC and CompareIC.
 void PatchInlinedSmiCode(Address address);
 
 } }  // namespace v8::internal
 
 #endif  // V8_IC_H_