Add VersionConstraint.difference().

See dart-lang/pubdart-lang/pub_semver#912

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2045803002 .
10 files changed
tree: 15fbd7850fdf0b67d1cc6fde9a3b99e47d79a680
  1. pkgs/