Add debug checks to catch crashes with WeakCell::cast().

src/objects-inl.h

 // Copyright 2012 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.
 //
 // Review notes:
 //
 // - The use of macros in these inline functions may seem superfluous
 // but it is absolutely needed to make sure gcc generates optimal
 // code. gcc is not happy when attempting to inline too deep.
 //
@@ skipping 2331 matching lines @@
 void FixedDoubleArray::FillWithHoles(int from, int to) {
   for (int i = from; i < to; i++) {
     set_the_hole(i);
   }
 }
 
 
 Object* WeakFixedArray::Get(int index) const {
   Object* raw = FixedArray::cast(this)->get(index + kFirstIndex);
   if (raw->IsSmi()) return raw;
+  DCHECK(raw->IsWeakCell());
   return WeakCell::cast(raw)->value();
 }
 
 
 bool WeakFixedArray::IsEmptySlot(int index) const {
   DCHECK(index < Length());
   return Get(index)->IsSmi();
 }
 
 
@@ skipping 5134 matching lines @@
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 #undef NOBARRIER_READ_BYTE_FIELD
 #undef NOBARRIER_WRITE_BYTE_FIELD
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_