Allow adding individual java/class files to classpath

[Devan-Kerman]

At the moment there is no way of adding a single java file or single class file to the classpath, nor can a virtual file system (like Jimfs) be used

[jamierocks]

I would have thought that adding individual Java source files (or Java class files) would work - interesting.

With regards to virtual file systems, that's an upstream consequence we've been aware of for a while - see GH-3.