NNBD support for generic functions where the result nullability depends on if an optional parameter is passed or not

A common pattern in Dart is to have an "orElse" callback on functions that needs to have a fallback behavior.

A concrete example is the `Iterable.firstWhere`:

```dart
final value = [1, 2, 3].firstWhere((i) => , orElse: () => )
```

Now, this is not specific to `Iterable.firstWhere` and is very common to Dart in general.\
NNBD doesn't cause any issue with `Iterable.firstWhere` specifically, as by default, it will throw if it needs a fallback but no `orElse` was provided.

The problem comes with a variant of that pattern, where instead of throwing, the default behavior of their `orElse` would be: `() => null`.

A typical implementation would be:

```dart
T doSomething({T orElse()}) {
// some logic

if (orElse != null) {
return orElse();
} else {
return null;
}
}
```

**The issue with that variant is that it is impossible to migrate to NNBD** without some inconvenient change.

This pattern gets stuck between two un-ideal solutions:
- either make `orElse` required:
```dart
T doSomething({required T orElse()});
```

This is not ideal because it creates a lot of duplicate code.\
We suddenly have to add tons of `orElse: () => null`.

- or make the return type **always** nullable:
```dart
T? doSomething({T orElse()});
```

This is not ideal either because if an `orElse` **is** specified, then the result may effectively never be null but will still be considered as nullable because of a language limitation.

So in the end, with NNBD enabled we either have pointless boilerplate or an incorrectly inferred type.

Start with the NNBD rules and the generic-function examples in the issue, then compare them with the Iterable.firstWhere optional orElse pattern. Determine whether the language can express a result whose nullability depends on whether the optional callback is supplied; done requires a documented language decision addressing both the boilerplate and always-nullable alternatives.

Written by the indexing model from the issue text.