Deprecate the source_maps span classes.

This also updates source_maps to take the new classes where possible.

BUG=19930
R=sigmund@google.com

Review URL: https://codereview.chromium.org//402843003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_maps@38524 260f80e4-7a28-3924-810f-c04153c831b5