Cleanup constant building in IrBuilder.

BUG=
R=floitsch@google.com

Review URL: https://codereview.chromium.org//657373003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41184 260f80e4-7a28-3924-810f-c04153c831b5
dart2js: Remove dead code.

R=johnniwinther@google.com

Review URL: https://codereview.chromium.org//663813004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41185 260f80e4-7a28-3924-810f-c04153c831b5