[dart2js] Clean up some nullabilities in static_type.
[dart2js] Clean up some nullabilities in static_type.

Change-Id: I731a4727ef3c74adbc6f7eaa9c05fc9cdeddad82
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/139962
Reviewed-by: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@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