Add an annotation that to indicate to missing_return that implicitly return null is ok

[Hixie]

It's common to define callbacks where returning null is a good default. For example, Flutter's onGenerateRoute is to return a route if it can handle the arguments, and return null otherwise, in which case other sources are consulted.

Right now, missing_return requires that we return null from all such callbacks explicitly.

It would be nice if we could annotate the typedef for these callbacks to say that rather than requiring a return, it's ok to be missing a return, and indeed, if the return null is the last statement in the method, it's better to just omit it. Maybe this would be a new lint that overrides missing_return for these annotated callbacks.

[srawlins]

@pq can you move this to the SDK repo? missing_return is an analyzer hint.

[pq]

This issue was moved to #35520