Object.oberve: assertions to narrow down flaky crashes with array length mutation.

src/objects.h

 // 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 4804 matching lines @@
   inline bool has_instance_call_handler() {
     return ((1 << kHasInstanceCallHandler) & bit_field()) != 0;
   }
 
   inline void set_is_extensible(bool value);
   inline bool is_extensible();
 
   inline void set_elements_kind(ElementsKind elements_kind) {
     ASSERT(elements_kind < kElementsKindCount);
     ASSERT(kElementsKindCount <= (1 << kElementsKindBitCount));
+    ASSERT(!is_observed() ||
+           elements_kind == DICTIONARY_ELEMENTS ||
+           elements_kind == NON_STRICT_ARGUMENTS_ELEMENTS ||
+           IsExternalArrayElementsKind(elements_kind));
     set_bit_field2((bit_field2() & ~kElementsKindMask) |
         (elements_kind << kElementsKindShift));
     ASSERT(this->elements_kind() == elements_kind);
   }
 
   inline ElementsKind elements_kind() {
     return static_cast<ElementsKind>(
         (bit_field2() & kElementsKindMask) >> kElementsKindShift);
   }
 
@@ skipping 4248 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_