This is new functionality in Robolectric:
http://robolectric.org/blog/2017/11/13/resources-for-real/

Note the latest release is 3.8, but it's unclear from the release notes if .arsc support is finished: https://github.com/robolectric/robolectric/releases

Loading resources is a very slow step in Robolectric's test start-up, so I'd expect the change to improve test runtimes.