This is just the first step; future CLs will add support for the new API to
various packages currently using the old one.

BUG=19930
R=sigmund@google.com

Review URL: https://codereview.chromium.org//381363002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_span@38360 260f80e4-7a28-3924-810f-c04153c831b5