Different behaviour in `dart:js` and `package:universal_html/js.dart`

[MatthaiosSait]

In https://github.com/dint-dev/universal_html/blob/0ea6592b88b0814edfce0ff10cb9a7159f87f9a6/lib/src/js.dart#L214 the Object is not nullable, whereas in

https://github.com/dart-lang/sdk/blob/234128e8d37d78f4c03405734eba3b05a6691ce5/sdk/lib/js/js.dart#L131

the value is nullable.

As an example, we cannot set the context with this operator, e.g. context['define'] = null;