Remove dead code in strong mode's warnings for null literals
Remove dead code in strong mode's warnings for null literals

This code should no longer be reachable. I think it was part of our early non-null primitive type experimentation. (Currently `null` literal is treated as bottom, so it should never be a downcast if casting from null.)

R=leafp@google.com

Review URL: https://codereview.chromium.org/2050443002 .

dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk