-observeValueForKeyPath now takes nullable parameters in new SDKs.
Fortunately, the test changes should still work with older SDKs because
we can override a T with a T?.
Swift SVN r27532
We have an SPI between the Swift compiler and Foundation based on the
SWIFT_SDK_OVERLAY_FOUNDATION_EPOCH preprocessor macro that allows us to
request the new API. rdar://20270080 tracks removing it.
Swift SVN r26475
This decreases total testing time by over a minute on my old Mac Pro.
It probably has much less effect on systems with fewer cores, but shouldn't
be any worse there.
Swift SVN r22745