Confusing singleWhereOrNull extension method

[jaripekkala]

https://github.com/dart-lang/collection/blob/v1.17.2/lib/src/iterable_extensions.dart#L296-L304

My proposal: Deprecate and delete singleWhereOrNull, and introduce more clear methods

Reasoning:
Many use the singleWhere method as replacement to firstWhere when the list should not contain more than one matching value, in cases when the logic would not work if code somewhere later changes in way that there are multiple matching values, so throwing error would make sense (developer need to go and handle the case).

But when also no matching value is acceptable, developer can mistakenly use singleWhereOrNull and have different behaviour than intended. These cases are not so commonly unit tested, as these cases are preventing mistakes in the future.

[lrhn]

I can see how the singleWhereOrNull behavior might not be what someone wants, if they're really asking for a zeroOrOneWhere: If there is none, give me null, if there is one, give me it, if there are more, something is seriously wrong, so throw.

I do think that elements.singleWhereOrNull(test) should behave like elements.where(test).singleOrNull, so maybe we just need to add another method to cover the zeroOrOneWhere/optionalSingleWhere case.