Fix the pkg/stack_trace tests.

On Windows they're now (correctly) using relative URIs as opposed to
relative paths.

Review URL: https://codereview.chromium.org//17157009

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/stack_trace@24205 260f80e4-7a28-3924-810f-c04153c831b5