This issue adds sourcesContent support for source maps: https://github.com/google/closure-compiler/issues/1890. This means that your source maps can include your source as well in one bundled file. This makes handling sourcemaps much easier for things like error tracking services. It could also simplify config for source mapping as everything is included in the source map and you don't need to specify relative paths, e.t.c.
This will need to wait for the next release of the Closure Compiler.