Remove computeMask from ConstantValue.

BUG=
R=karlklose@google.com

Review URL: https://codereview.chromium.org//756903005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42360 260f80e4-7a28-3924-810f-c04153c831b5
Disable type propagation for the dart backend.

R=johnniwinther@google.com

BUG= http://dartbug.com/21880

Review URL: https://codereview.chromium.org//807443002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42361 260f80e4-7a28-3924-810f-c04153c831b5