How to implement Dart with Spring MVC and Thymeleaf?
You will need to handle /packages/ requests separately, like your work-around. However, these hacks would only be needed for development when you are working with the actual .dart files.
When you deploy your app, you'll use either dart2js, dart2dart or, most likely, both. These tools produce a monolithic script file which does not rely on the external packages directory.
Since you only need the /packages/ directory for development, it is possible to set the the packages URL through a flag in Dartium. However, in my experience, this is an awkward solution since the flag applies to all Dart applications -- all Dart apps would need to fetch packages through the same URL scheme. It also makes it difficult to share your app with other Dartium installations.