Allow null in more 'special' type rules.
Allow null in more 'special' type rules.

We get extra precision from including argument types that produce errors.
e.g. {int} + {int,null} --> {int}.

Nice improvement on some numerical benchmarks where the result is used as an index where we can now use the builtin indexer.

Review URL: https://codereview.chromium.org/1493673003 .

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