The internal sun.tools.jar package is used here which leads to javac warning
http://hg.openjdk.java.net/jdk/jdk/file/241659594595/test/lib/RedefineClassHelper.java#l66

warning: sun.tools.jar.Main is internal proprietary API and may be removed in a future release

This can be avoided by using test/lib/ClassFileInstaller.java

    public static void main(String[] args) throws Exception {
        String manifest = "Premain-Class: RedefineClassHelper\nCan-Redefine-Classes: true\n";
        ClassFileInstaller.writeJar("redefineagent.jar", ClassFileInstaller.Manifest.fromString(manifest), "RedefineClassHelper");
    }