Adapt a generic type based on a potentially null value

We should be able to adapt an inferred generic type for based on a potentially `null` value.

## Example

Here is an example where a class stores an object and provides a particular transform of that object:
```dart
class MyView {
final T object;
final View Function(T object) _view;

MyView({
required this.object,
View Function(T object)? view,
}) : _view = view ?? ((v) => v);

View view() => _view(object);
}
```

This is currently showing the following error: `The return type 'T' isn't a 'View', as required by the closure's context.`

I am trying to provide a default identity transform when `view` passed to constructor is `null` (in that case, `View` should be `T`).

**Note**: Here `T` is not always `View`. For example, the object could be an `int` and the `view` could be `(n) => n.toString()`. However, when `view` is left `null` I would like `View` to be an `int`.

Dart should adapt the inferred type for `View` to `T` when `view` is `null`.

Start with the generic MyView constructor example in the issue and review Dart's generic type inference and null-aware context rules. Determine how inference should behave when view is null versus when it is a transforming function. Done means the intended inference behavior is specified clearly enough to support implementation and language tests.

Written by the indexing model from the issue text.