Fix flutter in preparation for implementing Dart's "Infer non-nullability from local boolean variables" (#72494)

When https://github.com/dart-lang/language/issues/1274 (Infer
non-nullability from local boolean variables) is implemented, flow
analysis will detect that code like this no longer needs to perform a
null check:

    final bool contextIsValid = focus != null && focus.context != null;
    ...
    if (contextIsValid) {
      ... focus! ... // Null check unnecessary
    }

To avoid a build failure due to the unnecessary null check, we need to
temporarily write it in a way that we can ignore it.  Once the feature
is complete and rolled into flutter, I'll remove the null check
entirely.