I think that must have changed recently (this year, maybe). I know something changed for sure because the method was always there, but it did not return a Future, like it should have, and did not have the optional Map. I don't recall seeing anything in the changelog.
I should fix it to use: https://api.dart.dev/stable/2.17.6/dart-html/Element/request...
I just never noticed because it wasn't causing any problems.