Better constant folding of length-loads in checked mode.

Handle redefinitions (Redefinition/AssertAssignable) when
checking for known constant length.

R=vegorov@google.com

Review URL: https://codereview.chromium.org//783683002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42132 260f80e4-7a28-3924-810f-c04153c831b5