`enum`s extending other types

#### Proposal

Allow the following as valid dart code:

```dart
enum MyEnum extends Foo { ... }
```

My understanding is that the `enum` keyword desugars to something like

```dart
class MyEnum extends _Enum { ... }
```

when instead, it could be

```dart
@EnumMacro()
class MyEnum implements Enum { ... }
```

With this change, you could make enumerated collections for widgets, colors, decorations, text styles… anything with a `const` constructor!

```diff
- enum NavBarButton {
+ enum NavBarButton extends StatelessWidget {
home(Icons.home),
leaderboard(Icons.leaderboard),
schedule(Icons.calendar_month),
profile(Icons.account_circle);

final IconData icon;
const NavBarButton(this.icon);

+ @override
- Widget get destination {
+ Widget build(BuildContext context) {
return NavigationDestination(icon: Icon(icon), label: name);
}
}
```

#### before

```dart
NavigationBar(destinations: [for (final button in NavBarButton.values) button.destination]);
```

#### after

```dart
NavigationBar(destinations: NavBarButton.values);
```

I'd love to see this implemented as part of the [code generation](https://github.com/dart-lang/language/issues/1565) release.

Start by reading the proposal and the linked code-generation issue (#1565), then review how Dart currently desugars enum declarations. Done means the shown enum-extending examples are valid Dart and the resulting values can be used as the demonstrated widgets or other const-constructible types.

Written by the indexing model from the issue text.