Change definition to class Optional<T extends Object>

[oprypin]

This prevents Optional from being declared with a nullable type. It is never useful to do so, because trying to create an instance with a null causes a run-time error anyway. So just confusing.

Internally we already applied this change and were quite easily able to clean up any existing occurrences.

[cbracken]

See for the last time we did this: #667.

If we've already done this internally though, that's great news and eliminates the main reason we backed this change out. It may be useful to conduct a quick use of open source projects to see what impact we can expect to users. Overall though, lgtm in principle from me.

[oprypin]

See: #667

A survey of the Google codebase suggests this would be a fairly extensive breaking change.

It is a breaking change but as I said I already applied this change there. So if Google codebase was a big part of the reason, it no longer applies.

[cbracken]

Apologies -- submitted the comment accidentally while I was still writing it. It's updated now.

[oprypin]

It may be useful to conduct a quick use of open source projects to see what impact we can expect to users.

Hm I haven't done such a survey before. Maybe if someone already has a setup for something like this, they could chip in.

[cbracken]

@yjbanov I think you've got experience doing this previously using BigQuery? If not, it's something the Flutter framework team has a lot of experience with. Will see if I can dig up a doc.