Revert "Handlify concat string and substring."

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index fb7f1ba4ad6ec370161703e0e8108b8d4543db7e..8bb603a985db87e6a8c0d78be94d081b74777f67 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -9303,6 +9303,14 @@ uint32_t StringHasher::ComputeUtf8Hash(Vector<const char> chars,
 }
 
 
+MaybeObject* String::SubString(int start, int end, PretenureFlag pretenure) {
+  Heap* heap = GetHeap();
+  if (start == 0 && end == length()) return this;
+  MaybeObject* result = heap->AllocateSubString(this, start, end, pretenure);
+  return result;
+}
+
+
 void String::PrintOn(FILE* file) {
   int length = this->length();
   for (int i = 0; i < length; i++) {