Before Null Safety, `_EventStreamSubscription.cancel()` used a trick
to run with synchronous timing even though it was typed to return a
`Future`. During the migration of the SDK to support Null Safety it
kept the synchronous timing in weak mode, but was changed to
asynchronous in sound mode so that the behavior matched the method
signature. In hindsight, changing the timing when opting into Null
Safety is problematic:
* A shared package has no control over what mode it runs in. Libraries
may be opted in and run their tests with sound null safety but the
apps they are used in could still be running in weak mode. This
results in library unit tests that behave differently than the
production app that deploys the code.
* This codepath can be triggered by EventTarget.dispatchEvent() from
dart:html which should have synchronous timings for the event
listeners before returning to the calling code. The asynchronous
timing when running with sound null safety is inconsistent with the
browser API.
This change reverses that migration decision and keeps the synchronous
timing in both modes. To support this in sound mode it returns a
special future value that is internal to the SDK and known to be used
for synchronous timing.
This change also removes the workaround introduced in DDC to avoid
warning/failing when `_EventStreamSubscription.cancel()` returned
null and the extra warnings/errors features were enabled in weak
null safety mode.
Change-Id: I6b08a2ada5b10120bea787ad59d1d58e6e181de5
Fixes: https://github.com/dart-lang/sdk/issues/44157
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/175323
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Srujan Gaddam <srujzs@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Nicholas Shahan
4c2edfd5b9