This is inspired by JDK-8149356, that handles the accidental commit of UseNewCode in the codebase. This flag is a development-time flag, and should not be used in committed code. Therefore, it might make sense to amend jcheck to test for these flags in the source code changes.

It can go like this: no new lines in the changeset can have UseNewCode in it, except for the actual UseNewCode declaration (e.g. if globals.hpp gets modified/reshuffled).