support part file completions

BUG=
R=paulberry@google.com

Review URL: https://codereview.chromium.org//815683002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42541 260f80e4-7a28-3924-810f-c04153c831b5
update completion cache key to include part directives

BUG=
R=paulberry@google.com

Review URL: https://codereview.chromium.org//812233003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42542 260f80e4-7a28-3924-810f-c04153c831b5