SI-5675 Restore former type of ConditionalWarning.warnings

In case the linkage error reported in SBT [1] proves to be
legitimate. To me, it looks like it just needs to recompile
the compiler-interface. But it this change can't hurt.

[1] https://groups.google.com/forum/?fromgroups=#!topic/scala-internals/ZpzayFCu_8E

Review by @lrytz