Rename mappedBy to map.

Retain a deprecated mappedBy for now.
Change return type of mappedBy, skip and take on List to Iterable.

BUG= http://dartbug.com/8063
BUG= http://dartbug.com/8064
BUG= http://dartbug.com/6739
BUG= http://dartbug.com/7982

Review URL: https://codereview.chromium.org//12086062

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/yaml@17899 260f80e4-7a28-3924-810f-c04153c831b5