The javadoc you're copying in wrong; it's left over from before I added the -saveSource option. Looking at the code in SourceSaver.java, the actual behavior is that if saveSourceOutput is null and the -saveSource option is true, then the source will be saved to the "extras" directory. Sorry about that! Fortunately I got it right in the command-line help.

Incidentally, a goal for open source GWT projects will probably be to save the source code somewhere within the web application's directory. That we we can serve it over HTTP and the browser's debugger can download it, assuming sourcemaps are set up properly. I don't know Maven so I don't know if there's anything special that needs to be done to enable this?