This means commands don't have to declare that they need.
It also ensures the pubspec isn't parsed until JSON output has been selected.

BUG=http://dartbug.com/20066, http://dartbug.com/20065
R=nweiz@google.com

Review URL: https://codereview.chromium.org//397843005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@38291 260f80e4-7a28-3924-810f-c04153c831b5