Add a hidden --no-package-symlinks flag.

This will help the editor test against the no-symlink behavior before
we roll it out to the world.

BUG=20253
R=danrubel@google.com, rnystrom@google.com

Review URL: https://codereview.chromium.org//427853002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@38702 260f80e4-7a28-3924-810f-c04153c831b5