In particular, this adds CHANGELOG entries for packages that have been
modified and fixes pub's barback dependency so it doesn't declare
compatibility on an uncut release.

This releases args, barback, http_parser, and shelf.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//506973003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@39563 260f80e4-7a28-3924-810f-c04153c831b5