Fix bug in source-maps parsing: parser failed when most information could be
inferred. This fixes bug http://dartbug.com/11782

R=dgrove@google.com

Review URL: https://codereview.chromium.org//19019006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_maps@24981 260f80e4-7a28-3924-810f-c04153c831b5