jrt-fs.jar classes are currently compiled with the BootJDK being used, this has a couple of issues:
- The classes are not reproducible between a standard and bootcycle build image, since the first is compiled by jdk-N-1 and the later jdk-N
- The jrt-fs classes in a standard product image are compiled by the BootJDK (typically jdk-N-1), which means from a "secure dev" perspective they are not compiled with the latest and greatest compiler with all the latest vulnerability fixes.

The only requirement on the jrt-fs classes is they are built jdk-8 compatible, so there is no reason they cannot be compiled with the jdk-N "interim" compiler as part of the standard build.