Inconsistent and incorrect nullability checks for JS interop return types and callback parameter types

[srujzs]

Our current handling of nullability of interop types are inconsistent:

sdk/pkg/dart2wasm/lib/js/util.dart

Line 115 in b4e30d1

if (returnType.isPotentiallyNonNullable) {

sdk/pkg/dart2wasm/lib/js/callback_specializer.dart

Line 36 in b4e30d1

if (!callbackParameterType.isPotentiallyNullable) {

In the callback parameter case, we add a null-check if the type is not potentially nullable, and in the return type case, we add a null-check if the type is potentially non-nullable. Both only handle some of the cases correctly. The right alternative for both cases is if the nullability is Nullability.nonNullable, place a null-check, and if it's Nullability.undetermined, cast.