Unnecessary copy eliminated
Profiling shows that ImmutableSet is faster than EnumSet in this case
Class made final and a call to getClass() eliminated
(Profiling results improved)
Avoiding rehashes (profiling results improved)
Creating physical files to avoid unnecessary work inside the PSI framework
forceResolveAll() is not called when compiler initializes built-ins
Optimization: no need to build a set of all supertypes, if we only need to check whether there's a nullable one
Optimization: Unneeded collection creation eliminated