Make implicit things explicit when loading scripts.

Explicitly:
Set current working directory
Set windows flag

Previously these were side effects of calling other functions.

Also use ' strings consistently.

R=iposva@google.com

Review URL: https://codereview.chromium.org//16368002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@24099 260f80e4-7a28-3924-810f-c04153c831b5