[Breaking change request] StreamSubscription.cancel can no longer return null

[lrhn]

Summary

The StreamSubscription.cancel method's return type is changed to Future<void>.

What is changing:

We change the return type of StreamSubscription.cancel to Future<void>.
The return type will be non-nullable in Null Safe code, and you are not allowed to return null.

Why is this changing?

The StreamSubscription.cancel method currently has a return type of Future (aka. Future<dynamic>). It allows you to return null as well, but this has been discouraged since the release of Dart 1, it's just not something the type system could prevent.

When migrating this method to Null Safety, we have decided to change the return type to Future<void> (non-nullable). That means that implementations of the StreamSubscription class can no longer return a null value, and you can no longer use the value of the returned future without casting the Future away from void first.
That is considered reasonable because the future is not intended to contain any useful value.

If a specific user of a specific stream subscription happens to know that that stream subscription's cancel returns a future with a useful value, then that should be reflected by having a sub-class of stream subscription with a different return type. (Or, as a hack, by casting the Future<void> to Future<dynamic> or similar before using it).

Expected impact

The type change disallows code which currently awaits the returned future and uses its value.
That code needs to cast the future to something else first, so the result won't have type void.

The non-null part won't be enforced until Null Safety, at which point tools will help find places where null is currently returned. That code should then return Future<Null>.value(null).

[franklinyow]

cc @Hixie @matanlurey @dgrove @vsmenon for review and approval.

[matanlurey]

LGTM

[leafpetersen]

ping on this.

[franklinyow]

Ping again @Hixie @vsmenon

[Hixie]

assuming there's no code seriously broken by this, lgtm

[vsmenon]

lgtm

[franklinyow]

Approved

[leafpetersen]

This has landed in bleeding edge.