The PrintSharedDictionary flag is not being used. It is just declared in globals.hpp.

./share/runtime/globals.hpp: product(bool, PrintSharedDictionary, false,

In 8u, the flag is for displaying the class names in a CDS archive as follows:

java -XX:+PrintSharedArchiveAndExit -XX:+PrintSharedDictionary

Currently in the mainline, one doesn't need to specify -XX:+PrintSharedDictionary in order to display the class names in a CDS archive.