InMemoryJavaFileManager has different behavior than standard java file manager.

The InMemoryJavaFileManager allows locations other than those specified in StandardLocation, whereas the standard manager will throw a null pointer exception if a non standard location is used.
[Please look at a simple unit test i made which illustrates this](https://github.com/jasonnn/compile-testing/blob/locations/src/test/java/com/google/testing/compile/NonStandardLocationsTest.java)

While it now seems kind of obvious to me that the "standard" manager would only support "standard" locations, it would be nice if compile-testing checked that it's backing file manager was actually able to create a FileObject for the specified location.