Add package root setting to analysis server API.

The command does not have any effect yet.

R=brianwilkerson@google.com

Review URL: https://codereview.chromium.org//672003002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41253 260f80e4-7a28-3924-810f-c04153c831b5