Handlify string concat and substring.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 30c3f63ea385178dd4ad7508afd388ac22d0515b..8661c100a6e02afb987c12ddc367aa3de9e98b38 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -8452,11 +8452,6 @@ 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);