Arduino Beta 1.5.8 cannot find libraries

This is less a request for help and more a resource for anybody suffering the same issue.
The Arduino Beta version 1.5.8 fails to resolve many of the libraries that worked in previous versions.
When attempting to compile a project from one of my repositories, I encountered the following errors:

fatal error: String.h: No such file or directory
fatal error: unistd.h: No such file or directory
 #include <unistd.h>

The only solution I could find was to revert to Arduino Beta version 1.5.6r2.