antiprioritize method

int antiprioritize (Version a, Version b)

Like prioritize, but lower version numbers are considered greater than higher version numbers.

This still considers prerelease versions to be lower than non-prerelease versions. Pub uses this when downgrading -- it chooses the lowest version but still excludes pre-release versions when possible.

Implementation 

static int antiprioritize(Version a, Version b) {
  if (a.isPreRelease && !b.isPreRelease) return -1;
  if (!a.isPreRelease && b.isPreRelease) return 1;

  return b.compareTo(a);
}