Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/heap-inl.h

Index: src/heap-inl.h
diff --git a/src/heap-inl.h b/src/heap-inl.h
index 063cf30ff3cd178b4c7efda22fcc2447252e90c8..f6525ae307186f54d43d01bbe0574a3e51da0653 100644
--- a/src/heap-inl.h
+++ b/src/heap-inl.h
@@ -273,7 +273,7 @@ MaybeObject* Heap::NumberFromInt32(
     int32_t value, PretenureFlag pretenure) {
   if (Smi::IsValid(value)) return Smi::FromInt(value);
   // Bypass NumberFromDouble to avoid various redundant checks.
-  return AllocateHeapNumber(FastI2D(value), pretenure);
+  return AllocateHeapNumber(FastI2D(value), IMMUTABLE, pretenure);
 }
 
 
@@ -284,7 +284,7 @@ MaybeObject* Heap::NumberFromUint32(
     return Smi::FromInt(static_cast<int32_t>(value));
   }
   // Bypass NumberFromDouble to avoid various redundant checks.
-  return AllocateHeapNumber(FastUI2D(value), pretenure);
+  return AllocateHeapNumber(FastUI2D(value), IMMUTABLE, pretenure);
 }