Allow smis for singleton types

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 7d204af2b4f6ecae63ff085f312b7198259e149e..d3daaf79c854a9653989fc57db63bccbb678be0c 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -939,6 +939,10 @@ class Heap {
   // Please note this does not perform a garbage collection.
   MUST_USE_RESULT MaybeObject* AllocateJSGlobalPropertyCell(Object* value);
 
+  // Allocate Box.
+  MUST_USE_RESULT MaybeObject* AllocateBox(Object* value,
+                                           PretenureFlag pretenure);
+
   // Allocates a fixed array initialized with undefined values
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
   // failed.