Revert "Handlify concat string and substring."

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index f793cc332d6e50febe99fad1de851bf3f10a85a8..ea70f076c2ac46ee8e21448cd48d96060ba72baf 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1068,6 +1068,25 @@ class Heap {
       Object* stack_trace,
       Object* stack_frames);
 
+  // Allocates a new cons string object.
+  // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
+  // failed.
+  // Please note this does not perform a garbage collection.
+  MUST_USE_RESULT MaybeObject* AllocateConsString(String* first,
+                                                  String* second);
+
+  // Allocates a new sub string object which is a substring of an underlying
+  // string buffer stretching from the index start (inclusive) to the index
+  // end (exclusive).
+  // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
+  // failed.
+  // Please note this does not perform a garbage collection.
+  MUST_USE_RESULT MaybeObject* AllocateSubString(
+      String* buffer,
+      int start,
+      int end,
+      PretenureFlag pretenure = NOT_TENURED);
+
   // Allocate a new external string object, which is backed by a string
   // resource that resides outside the V8 heap.
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation