Fix lingering references to the old Span in yaml. R=rnystrom@google.com BUG= Review URL: https://codereview.chromium.org//413403002 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/yaml@38656 260f80e4-7a28-3924-810f-c04153c831b5