* Make dart wrappable classes use only one native field

It appears like the second native field is never really used for
anything: It is set, cleared and verified it was cleared, but that is
it.

=> The only native field that is necessary is the `this` pointer of the
   equivalent C++ object.
=> The [DartWrapperInfo] object which was stored as second native field
   is always accessible via the virtual `GetDartWrapperInfo()` method on
   `this`.