Fix issue 7215 by relativising a path before checking its components.

This includes a couple more miscellaneous fixes.

BUG=7215

Review URL: https://codereview.chromium.org//11528020

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@16002 260f80e4-7a28-3924-810f-c04153c831b5