Most of these tests used this directive as a way to opt in to a new
language feature, and most of those features are already stable for a
long time, so no opt-in is needed. Some other tests used the directive
to opt out from a language feature, replace those by the `LANGUAGE`
directive. One test used the directive to test behavior that actually
depended on the API version; use `API_VERSION` directive there instead.
This annotation is currently internal because we only commit to its
support for our own libraries. It will be used to change JVM package
names of declarations in JDK-specific stdlib additions (now called
kotlin-stdlib-jre7/8), both to preserve source compatibility of the old
Kotlin code and to solve the split package problem (KT-19258)
Alexander Udalov