Make StringScannerException extend SpanFormatException.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//343043002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/string_scanner@37619 260f80e4-7a28-3924-810f-c04153c831b5
3 files changed
tree: e625ebdd258c7c35540bea811c916c09abc70c07
  1. pkgs/