Redo "Constant fold charCodeAt via constant system"

TBR=johnniwinther@google.com
BUG=

Review URL: https://codereview.chromium.org//682243004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@41384 260f80e4-7a28-3924-810f-c04153c831b5
7 files changed