The plugin currently has a concatenation step before sending the file into the magnification.
This behaviour is at odds with the Closure Compiler. The compiler expects to receive all the files separately, one to make sense of @Private annotations and also for the the .map file.
If you compile with the latest version of the plugin, the final .map file will point to a transient file called .tmp that does not exist on the final output.
For an example of the configuration you can see this file: