Use mutable heapnumbers to store doubles in fields.

test/cctest/test-heap.cc

Index: test/cctest/test-heap.cc
diff --git a/test/cctest/test-heap.cc b/test/cctest/test-heap.cc
index 1de24ce88d4b62857859757698590d06133bf312..2925247ce945bd54957d956966f6935431055c8e 100644
--- a/test/cctest/test-heap.cc
+++ b/test/cctest/test-heap.cc
@@ -2379,7 +2379,7 @@ TEST(Regress2211) {
     // Check size.
     DescriptorArray* descriptors = internal_obj->map()->instance_descriptors();
     ObjectHashTable* hashtable = ObjectHashTable::cast(
-        internal_obj->FastPropertyAt(descriptors->GetFieldIndex(0)));
+        internal_obj->RawFastPropertyAt(descriptors->GetFieldIndex(0)));
     // HashTable header (5) and 4 initial entries (8).
     CHECK_LE(hashtable->SizeFor(hashtable->length()), 13 * kPointerSize);
   }