Properly handle empty stack traces in package:stack_trace.

R=sigmund@google.com

Review URL: https://codereview.chromium.org//18170002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/stack_trace@24571 260f80e4-7a28-3924-810f-c04153c831b5