This reverts commit c97f7b7fad.
Reason for revert: Still breaking google3 (b/236665701)
Original change's description:
> Make `nullFuture` be per-zone.
>
> We introduced a `nullFuture` during the null-safety migration where
> we changed some methods to no longer allow returning `null`,
> and they therefore had to return a `Future`.
> That affected timing, because returning `null` was processed
> synchronously, and that change in timing made some tests fail.
> Rather that fix the fragile tests, we made the function return
> a recognizable future, a canonical `Future<Null>.value(null)`,
> and then recognized it and took a synchronous path for it.
>
> That caused other issues, because the future was created in the
> root zone. (Well, originally, it was created in the first zone
> which needed one, that was worse. Now it's created in the root zone.)
> Some code tries to contain asynchrony inside a custom zone, and
> then the get a `nullFuture` and calls `then` on it, and that
> schedules a microtask in the root zone.
> (It should probably have used the listener's zone, and not store
> a zone in the future at all, but that's how it was first done,
> and now people rely on that behavior too.)
>
> This change creates a `null` future *per zone* (lazily initialized
> when asked for). That should be sufficient because the code recognizing
> a returned `null` future is generally running in the same zone,
> but if any other code gets the `nullFuture`, it will be in the
> expected zone for where it was requested.
>
> This is a reland of commit a247b158d6
>
> Change-Id: Ieec74d6f93c57175c357ec18889144635f5bdca6
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/249490
> Commit-Queue: Lasse Nielsen <lrn@google.com>
> Reviewed-by: Erik Ernst <eernst@google.com>
TBR=lrn@google.com,eernst@google.com,nbosch@google.com
Change-Id: I870285b03ec05803c5aaa6b66f9a6e6ea360d732
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/249609
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Emmanuel Pellereau <emmanuelp@google.com>
Reviewed-by: Emmanuel Pellereau <emmanuelp@google.com>
Emmanuel Pellereau
fd590c54c2