Add publishTo to pubspec and use it in pub lish.

RELNOTE=Pubspecs can have a "publishTo" field which is either a URL to
  specify the default server that publishing should use or "none" to
  specify that the package is private and cannot be published.
BUG=https://code.google.com/p/dart/issues/detail?id=20046
R=nweiz@google.com

Review URL: https://codereview.chromium.org//417043005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@38880 260f80e4-7a28-3924-810f-c04153c831b5