the idea is that often times in code, you will see something like this:...

src/heap.cc

Index: src/heap.cc
===================================================================
--- src/heap.cc	(revision 3015)
+++ src/heap.cc	(working copy)
@@ -1664,7 +1664,9 @@
 }
 
 
-Object* Heap::AllocateConsString(String* first, String* second) {
+Object* Heap::AllocateConsString(String* first,
+                                 String* second,
+                                 PretenureFlag pretenure) {
   int first_length = first->length();
   if (first_length == 0) return second;
 
@@ -1687,7 +1689,7 @@
     ASSERT(first->IsFlat());
     ASSERT(second->IsFlat());
     if (is_ascii) {
-      Object* result = AllocateRawAsciiString(length);
+      Object* result = AllocateRawAsciiString(length, pretenure);
       if (result->IsFailure()) return result;
       // Copy the characters into the new object.
       char* dest = SeqAsciiString::cast(result)->GetChars();
@@ -1699,7 +1701,7 @@
       for (int i = 0; i < second_length; i++) *dest++ = src[i];
       return result;
     } else {
-      Object* result = AllocateRawTwoByteString(length);
+      Object* result = AllocateRawTwoByteString(length, pretenure);
       if (result->IsFailure()) return result;
       // Copy the characters into the new object.
       uc16* dest = SeqTwoByteString::cast(result)->GetChars();
@@ -1721,12 +1723,17 @@
       : long_cons_string_map();
   }
 
-  Object* result = Allocate(map, NEW_SPACE);
+  AllocationSpace space =
+      (pretenure == TENURED) ? OLD_POINTER_SPACE : NEW_SPACE;
+  if (map->instance_size() > MaxObjectSizeInPagedSpace()) space = LO_SPACE;
+  Object* result = Allocate(map, space);
   if (result->IsFailure()) return result;
-  ASSERT(InNewSpace(result));
+  ASSERT(pretenure == TENURED ? !InNewSpace(result) : InNewSpace(result));
   ConsString* cons_string = ConsString::cast(result);
-  cons_string->set_first(first, SKIP_WRITE_BARRIER);
-  cons_string->set_second(second, SKIP_WRITE_BARRIER);
+  WriteBarrierMode write_barrier_mode =
+      (pretenure == TENURED) ? UPDATE_WRITE_BARRIER : SKIP_WRITE_BARRIER;
+  cons_string->set_first(first, write_barrier_mode);
+  cons_string->set_second(second, write_barrier_mode);
   cons_string->set_length(length);
   return result;
 }