Be lazier when capturing stack traces.

It turns out that capturing stack traces is cheap; converting them to strings is
what's so expensive.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//14647003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/stack_trace@22205 260f80e4-7a28-3924-810f-c04153c831b5
2 files changed
tree: 8aa03c452c2875709dfa79aa6f8dc23b6c8d4920
  1. pkgs/