Make pubspec.lock keep relative path if it is originally specified as relative in pubspec.yaml (issue 11441).

BUG=11441
R=rnystrom@google.com

Review URL: https://codereview.chromium.org//34803006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@30060 260f80e4-7a28-3924-810f-c04153c831b5