It seems that the library is being moved to Github (it seems the infobox.js wasn’t moved yet), see the announcement on main page: https://code.google.com/p/google-maps-utility-library-v3/
But still, the problem with your code is that it’s not a good practise to reference code from code.google.com svn repository. It’s like referencing a code from Github, it can be changed/moved/removed any time. You should either download the code and include it in your project as .js file or host it yourself on some CDN server.
UPDATE
The google utility library (including infobox) is hosted here on github now. As said before, it’s not mean to be referenced from there in projects.