Handlify concat string and substring.

src/objects.h

 // 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
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   FlatContent GetFlatContent();
 
   // Returns the parent of a sliced string or first part of a flat cons string.
   // Requires: StringShape(this).IsIndirect() && this->IsFlat()
   inline String* GetUnderlying();
 
   // Mark the string as an undetectable object. It only applies to
   // ASCII and two byte string types.
   bool MarkAsUndetectable();
 
-  // Return a substring.
-  MUST_USE_RESULT MaybeObject* SubString(int from,
-                                         int to,
-                                         PretenureFlag pretenure = NOT_TENURED);
-
   // String equality operations.
   inline bool Equals(String* other);
   bool IsUtf8EqualTo(Vector<const char> str, bool allow_prefix_match = false);
   bool IsOneByteEqualTo(Vector<const uint8_t> str);
   bool IsTwoByteEqualTo(Vector<const uc16> str);
 
   // Return a UTF8 representation of the string.  The string is null
   // terminated but may optionally contain nulls.  Length is returned
   // in length_output if length_output is not a null pointer  The string
   // should be nearly flat, otherwise the performance of this method may
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     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_