Specifically, when run with "--trace" or "--verbose", pub will capture its stack
chains and report them along with errors. Otherwise it should behave identically
to before.

R=rnystrom@google.com
BUG=8842

Review URL: https://codereview.chromium.org//113453005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@31176 260f80e4-7a28-3924-810f-c04153c831b5