Support string add in crankshaft:

src/arm/code-stubs-arm.h

 // Copyright 2010 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 140 matching lines @@
                        Label* gc_required,
                        SmiCodeGenerateHeapNumberResults heapnumber_results);
   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 GenerateAddStrings(MacroAssembler* masm);
   void GenerateCallRuntime(MacroAssembler* masm);
 
   void GenerateHeapResultAllocation(MacroAssembler* masm,
                                     Register result,
                                     Register heap_number_map,
                                     Register scratch1,
                                     Register scratch2,
                                     Label* gc_required);
@@ skipping 192 matching lines @@
 
   bool NeedsImmovableCode() { return true; }
 
   const char* GetName() { return "DirectCEntryStub"; }
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_ARM_CODE_STUBS_ARM_H_