Expression Widgets has an integrated build system. This build system is an experimental project and should be evaluated as a contribution to the platform in terms of ideas and needs/challenges. The Jetpack Prototype provides a mechanism to register one JavaScript file as being the developer application. It was found that this does not help much as the size of the application gets bigger.
So it was prefered to keep all the files as separated and join them, packaging them as a single file. The build system is no more than that except it has an option where some JS can be added at the end of everything as a means to enable/disable some integrated debugging options.