BUG=https://code.google.com/p/dart/issues/detail?id=5391
RELNOTE=Add "pub cache repair" command to re-install cached packages.
R=nweiz@google.com

Review URL: https://codereview.chromium.org//228703006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@35111 260f80e4-7a28-3924-810f-c04153c831b5