In particular, this makes barback and the "pub lish" validators use gitignore
rules.

R=rnystrom@google.com
BUG=12102, 11198

Review URL: https://codereview.chromium.org//342553002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@37469 260f80e4-7a28-3924-810f-c04153c831b5