Handlify concat string and substring.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index e8c985048455e406c77c6b4c6814261f2cb7ab41..f50cb9ce2b3311acd92d8d5304e32e25f8b55903 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -8481,11 +8481,6 @@ class String: public Name {
   // 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);