Don't allow running global executables outside of bin.

BUG=https://code.google.com/p/dart/issues/detail?id=19993
R=nweiz@google.com

Review URL: https://codereview.chromium.org//396773003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@38353 260f80e4-7a28-3924-810f-c04153c831b5