Confusion with extension types and `implements`

First, I apologize in advance in case I misunderstand the meaning of `implements` in extension types. However, I find that it is confusing, considering how `implements` work in classes. This is mainly due to the fact that in classes, `implements` means that a class must follow a specified interface or "shape" of an object. However, in extension types, `implements` (I think) means "to expose" the interface of the underlying object. For example:

```dart
extension type const ImmutableList(T _) implements Iterable {
T operator[](int index) => _[index];
int get length => _.length; /// This is not necessary because of the implements clause.
}
```

So what's the idea with implements? Is it supposed to expose the underlying API of the object? Is it supposed to ensure that the object obeys its superinterface by exposing needed elements?

Start with the extension type example in the issue and compare its use of `implements` with the class meaning described there. Determine and document whether the clause exposes the underlying API, enforces a superinterface, or serves another purpose; done means the semantics and distinction are clearly resolved.

Written by the indexing model from the issue text.