If you're using the Windows client of AppBuilder or its extension for Visual Studio, I'm afraid you won't be able to load the source code using source mapping. I logged this issue in our internal issue tracker for further investigation.
In the meantime, you can use AppBuilder's In-Browser client
where source mapping works as expected. Should you have any issues with it, post them here so I can investigate and advice you further.
I also want to mention that debugging minified code on a mobile device is hard (at best you'll have to serve the mappings on a web server and manually add them through the DevTools context menu). This is why it is recommended that you don't minify your source files until your app is ready for publishing.