- Parse them in pubspec.
- Pass them to solver.
- Warn when they are used.
- Error on publish.

BUG=https://code.google.com/p/dart/issues/detail?id=8566
R=nweiz@google.com

Review URL: https://codereview.chromium.org//74013007

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@30376 260f80e4-7a28-3924-810f-c04153c831b5