swift234

[Accessibility] Observing overrides must be able to access the base setter.

Otherwise, the subclasser isn't really supposed to know that the base
property is internally settable. Overriding such a property isn't likely
to do much good (because the base class won't call your overridden setters),
but allowing observation leaks information about the base class.

<rdar://problem/17632360>

Swift SVN r20440