We are migrating Guava to use these annotations rather than jsr305's @nullable.
We can't use the Checker Framework's _@Nullable_ yet because we promise compatibility with Java 7, which doesn't support type annotations.
This is related to but distinct from https://youtrack.jetbrains.com/issue/KT-21408, which is about a different jsr305 annotation we use, @ParametersAreNonnullByDefault.

I've also updated some docs to mention Kotlin's existing support for the Checker Framework _@NonNull_.

NOTE: This commit does not yet run the new tests (though I've found a hacky way to confirm that they will indeed pass). I am unsure how to update files like ForeignAnnotationsTestGenerated.java that say they are generated with org.jetbrains.kotlin.generators.tests.TestsPackageorg.jetbrains.kotlin.generators.tests.TestsPackage (and so testAllFilesPresentInTests is failing for them). Please advise me how to update those files. Thanks.