Replace ResolvedNode by ConstantExpression.

BUG=http://dartbug.com/21493
R=sigurdm@google.com

Review URL: https://codereview.chromium.org//716913005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41709 260f80e4-7a28-3924-810f-c04153c831b5