Use mutable heapnumbers to store doubles in fields.

test/cctest/test-heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2361 matching lines @@
     if (i == 1) obj->SetHiddenValue(v8_str("key string"), value);
 
     // Check values.
     CHECK_EQ(hash,
              internal_obj->GetHiddenProperty(heap->identity_hash_string()));
     CHECK(value->Equals(obj->GetHiddenValue(v8_str("key string"))));
 
     // 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);
   }
 }
 
 
 TEST(IncrementalMarkingClearsTypeFeedbackCells) {
   if (i::FLAG_always_opt) return;
   CcTest::InitializeVM();
   v8::HandleScope scope(CcTest::isolate());
@@ skipping 672 matching lines @@
   }
   // An entire block of handles has been filled.
   // Next handle would require a new block.
   ASSERT(data->next == data->limit);
 
   DeferredHandleScope deferred(isolate);
   DummyVisitor visitor;
   isolate->handle_scope_implementer()->Iterate(&visitor);
   deferred.Detach();
 }