Elide write barriers and remove some heap_object->GetHeap() calls on

src/heap-inl.h

Index: src/heap-inl.h
===================================================================
--- src/heap-inl.h	(revision 10171)
+++ src/heap-inl.h	(working copy)
@@ -125,7 +125,8 @@
     if (!maybe_result->ToObject(&result)) return maybe_result;
   }
 
-  reinterpret_cast<HeapObject*>(result)->set_map(map);
+  // String maps are all immortal immovable objects.
+  reinterpret_cast<HeapObject*>(result)->set_map_unsafe(map);
   // Set length and hash fields of the allocated string.
   String* answer = String::cast(result);
   answer->set_length(str.length());