This also makes "pub cache repair" repair activated packages, including updating
the binstub format.

R=rnystrom@google.com
BUG=21463

Review URL: https://codereview.chromium.org//745153002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42055 260f80e4-7a28-3924-810f-c04153c831b5