Fix for issue 20125 - improved error messages

R=scheglov@google.com

Review URL: https://codereview.chromium.org//712663002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41613 260f80e4-7a28-3924-810f-c04153c831b5
Fix source_span tests.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//712473002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41614 260f80e4-7a28-3924-810f-c04153c831b5