Summary:
Kotlin has an experimental support for [JSR-305 custom nullability
qualifiers](https://github.com/Kotlin/KEEP/blob/master/proposals/jsr-305-custom-nullability-qualifiers.md).
Annotating Nullsafe in a special way makes kotlinc recognize it as
such custom nullability qualifier and therefore treat types coming
from Nullsafe Java classes **not** as platform types, but rather
proper nonnull/nullable, which affects:
1. Generated bytecode (more thorough null-checks).
2. Type inference in the IDE.
NOTE re: p.1: one might expect that with properly annotated Java code
Kotlin would avoid inserting runtime checks. This is not how
Kotlin-Java interop works - in reality Kotlin does even more runtime
checking for Java code annotated as Nonull, which IMO is a good
thing, since you can't trust Java anyway.
Reviewed By: mityal
Differential Revision: D21278440
fbshipit-source-id: d0598738a
Artem Pianykh