This structure looked familiar to me:

  +-------+
  |   *---+-> hidden class (aka map)
  +-------+
  |   *---+-> out-of-object properties storage
  +-------+
  |   *---+-> elements storage
  +-------+
  |   1   +-+
  +-------+ |
  |   2   | |
  +-------+  > N slots reserved for in-object properties
  ~~~~~~~~~ |
  +-------+ |
  |   N   +-+
  +-------+

That's because it's very close to PHP 7's implementation of objects (PHP has "traditional" class-based OOP):

  zend_object:
  +-------------+-------------+------------+-----------------------------------------+
  | gc_info     | flags       | IS_OBJECT  | refcount (32-bit)                       |
  +-------------+-------------+------------+-----------------------------------------+
  | UNUSED (32-bit)                        | handle (32-bit) FIXME                   |
  +----------------------------------------+-----------------------------------------+
  | ce - pointer to zend_class_entry (64-bit or 32-bit)                              |
  +----------------------------------------------------------------------------------+
  | handlers - pointer to object handlers (64-bit or 32-bit)                         |
  +----------------------------------------------------------------------------------+
  | properties - pointer to HashTable of dynamic properties (64-bit or 32-bit)       |
  +----------------------------------------------------------------------------------+
  | guards - pointer to HashTable used for recursion protection (64-bit or 32-bit)   |
  +----------------------------------------------------------------------------------+
  | properties_table[0] - embedded cell of the first declared property (128-bit)     |
  +----------------------------------------------------------------------------------+
  | ...                                                                              |
  +----------------------------------------------------------------------------------+
  | properties_table[N] - embedded cell of the last declared property (128-bit)      |
  +----------------------------------------------------------------------------------+
  | optional user data of internal classes (variable size)                           |
  +----------------------------------------------------------------------------------+
   63         48 47         40 39        32 31                                      0

(source: https://wiki.php.net/phpng-int#is_object)

PHP 7 objects contain N slots for properties defined in the class, plus a hashtable for any additional properties, just like V8 objects.

I guess it's an obvious way to design it.