Clean up various ways we find paths in pub.

This makes it so that it's never necessary to pass in DART_SDK.

Review URL: https://codereview.chromium.org//14449004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/stack_trace@21914 260f80e4-7a28-3924-810f-c04153c831b5