Tighter types on dart2js truncating divide

Always returns 'int'.

BUG=
R=floitsch@google.com

Review URL: https://codereview.chromium.org//718333002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41726 260f80e4-7a28-3924-810f-c04153c831b5