Expose interfaces for enums and enum items for generic programming

This is the test case I want to satisfy:
```dart
enum TestEnum { anda, murgi }
void main() {
group("EnumConverter should convert to and back between enum and string", () {
// setup
EnumConverter testEnumConverter = EnumConverter();
test("toJson(TestEnum.anda) should return 'anda'", () {
expect(testEnumConverter.toJson(TestEnum.anda), equals("anda"));
});
});
}
```
highlight:
```dart
expect(testEnumConverter.toJson(TestEnum.anda), equals("anda"));
```

I want to write a converter that can take the type of enum and it is impossible to get the enum interface in the type.

In my ideal case scenario, a solution like this should have worked:

```dart
abstract class EnumInterface {
List get values;
}

abstract class EnumItemInterface {
int get index;
String toString();
}

class EnumConverter implements JsonConverter {
@override
T fromJson(String json) {
return (T as EnumInterface)
.values
.firstWhere((element) => element.toString().endsWith(json)) as T;
}

@override
String toJson(T json) {
// just make sure there are no '.' s in the enum class name or this will fail.
return (json as EnumItemInterface).toString().split('.')[1];
}
}
```

Unfortunately, casts like `JSON as EnumItemInterface` are not as mindless in dart and I am met with an error:
```
type 'TestEnum' is not a subtype of type 'EnumItemInterface' in typecast
```

Enums and items in the enums already implement a fixed interface. Just exposing those interfaces in the SDK will give me enough to work with, enabling generic programming with enum classes.

A way to impose interfaces on top of classes that we know have that interface but are cast as not having it would also be cool.

Start with the embedded Dart enum and EnumConverter test case, then read the proposed EnumInterface and EnumItemInterface definitions. Determine whether the SDK or language specification can expose the existing enum interfaces for generic access. Done means an accepted design and corresponding specification or SDK changes that support the demonstrated conversion use case.

Written by the indexing model from the issue text.