Cleanup API of StdioServerSocket to prevent users from passing in a non-null package root.

R=scheglov@google.com

Review URL: https://codereview.chromium.org//754973002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41943 260f80e4-7a28-3924-810f-c04153c831b5