Experimental and UseExperimental can only be used as annotations or
qualifiers (to allow "Experimental.Level.*"); experimental markers can
only be used as annotations or qualifiers, or as left-hand side of a
::class literal in arguments to UseExperimental/WasExperimental.
This is needed because we're not yet sure of the design of this feature
and would like to retain the possibility to drop these declarations
(Experimental, UseExperimental) altogether. If they were going to be
used as types, it would be problematic because we can't simply delete
something from stdlib, should deprecate it first. With this change,
these declarations can only be used if the user has opted into using the
experimental API somehow (for example, with
`-Xuse-experimental=kotlin.Experimental`), so we won't stop conforming
to our deprecation policy if we decide to remove these declarations in
the future
Alexander Udalov