[heap] Assert that there's no recodred slot corresponding to unboxed double field.

src/objects.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/objects.h"
 
 #include <cmath>
 #include <iomanip>
 #include <memory>
 #include <sstream>
@@ skipping 3420 matching lines @@
     // Can't use JSObject::FastPropertyAtPut() because proper map was not set
     // yet.
     if (new_map->IsUnboxedDoubleField(index)) {
       DCHECK(value->IsMutableHeapNumber());
       object->RawFastDoublePropertyAtPut(index,
                                          HeapNumber::cast(value)->value());
       if (i < old_number_of_fields && !old_map->IsUnboxedDoubleField(index)) {
         // Transition from tagged to untagged slot.
         heap->ClearRecordedSlot(*object,
                                 HeapObject::RawField(*object, index.offset()));
+      } else {
+        DCHECK(!heap->HasRecordedSlot(
+            *object, HeapObject::RawField(*object, index.offset())));
       }
     } else {
       object->RawFastPropertyAtPut(index, value);
     }
   }
 
 
   // If there are properties in the new backing store, trim it to the correct
   // size and install the backing store into the object.
   if (external > 0) {
@@ skipping 16991 matching lines @@
       // depend on this.
       return DICTIONARY_ELEMENTS;
     }
     DCHECK_LE(kind, LAST_ELEMENTS_KIND);
     return kind;
   }
 }
 
 }  // namespace internal
 }  // namespace v8