Jigsaw uses a new image file format instead of rt.jar.
http://openjdk.java.net/jeps/220

But javac needs to be able to support system classes in .jar files for -target <= 8, and as such, we need to be able to generate lookalike rt.jar files for testing.

It is proposed to add a new method to the JarTask in the ToolBox test framework.
    extract(JavaFileManager fm, Location l, String... paths)
The paths can be a specific path (e.g. java/lang/Object.class) or the contents of a package (e.g. java/lang/*) or the contents of a package and its subpackages (e.g. java/lang/**)