Always capture stack traces when running pub tests.

Now that stack_trace lazily parses stacks, this should be fast enough to be
usable.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//14706002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@22217 260f80e4-7a28-3924-810f-c04153c831b5