Don't restrict type on Setlet.remove.

Review URL: https://codereview.chromium.org//730023002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41772 260f80e4-7a28-3924-810f-c04153c831b5
Documentation to follow up on CLs 714843002 and 719443002.

R=johnniwinther@google.com

Review URL: https://codereview.chromium.org//730013003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41773 260f80e4-7a28-3924-810f-c04153c831b5