This is useful for ensuring that a package's dependencies' lower bounds are
correct.

R=rnystrom@google.com
BUG= https://code.google.com/p/dart/issues/detail?id=15670

Review URL: https://codereview.chromium.org//365993007

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@38220 260f80e4-7a28-3924-810f-c04153c831b5