This releases barback 0.14.2, which supports both SourceSpan and Span.
It will be followed up with barback 0.15.0, which will remove support
for Span.

barback 0.14.1 will be added to third_party/pkg in a follow-up CL.

R=rnystrom@google.com
BUG=19930

Review URL: https://codereview.chromium.org//423823010

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@38712 260f80e4-7a28-3924-810f-c04153c831b5