[dart2js] Always treat Never as a bottom type.
[dart2js] Always treat Never as a bottom type.

Previously, we were only treating it as a bottom type with strong NNBD,
but weak NNBD can still contain Never.

Change-Id: I88f1aac3d228918f82e7e75d01531df802613ddb
Bug: https://github.com/dart-lang/sdk/issues/41970
Fixes: https://github.com/dart-lang/sdk/issues/41970
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/148940
Reviewed-by: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk