Type check field assignments during constant evaluation.

BUG=dartbug.com/16391
R=scheglov@google.com

Review URL: https://codereview.chromium.org//644283002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41062 260f80e4-7a28-3924-810f-c04153c831b5