Don't cache path Context based on cwd, as cwd involves a system-call to compute.

Instead, delay evaluation of cwd to the actual use of it.

BUG=
R=rnystrom@google.com

Review URL: https://codereview.chromium.org//429173002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@38844 260f80e4-7a28-3924-810f-c04153c831b5