See dart-lang/ai#171

We need this to roll into the SDK before we can test it.

I think it's safe to assume that the flag exists if the user has configured a log file (eg. don't worry about trying to figure out exactly which version number has support for this flag.. few people will set this flag and those that do will almost certainly be on some future stable version that includes the flag, or bleeding edge that will have it once we implement this).