Add tree shaker support for setters and field writes.

R=scheglov@google.com

Review URL: https://codereview.chromium.org//652613004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41229 260f80e4-7a28-3924-810f-c04153c831b5
Issue 21372. "Convert into block body" and "Convert into expression body" for factory constructors.

R=paulberry@google.com
BUG= https://code.google.com/p/dart/issues/detail?id=21372

Review URL: https://codereview.chromium.org//664403002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41230 260f80e4-7a28-3924-810f-c04153c831b5