Make stack frames without columns print correctly.

This works around issue 14233.

R=alanknight@google.com

Review URL: https://codereview.chromium.org//29323004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/stack_trace@28884 260f80e4-7a28-3924-810f-c04153c831b5