Fix internal constructor in context.
Fix internal constructor in context.

BUG=
R=lrn@google.com

Review URL: https://codereview.chromium.org//434393002

git-svn-id: d21bfdc9f98bb697a501ab295d9b4211dc860b14@38845 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/path