Add possibility to access Enum values by its Type

Currently we can access enum values in such a way:
```dart
enum Foo {
bar,baz
}

void main() {
print(Foo.values);
}
```

But this becomes problem when working with generics:
```dart
E findEnum(final String toString) {
List values = []; //?How to get the values from E?
return values.firstWhere((element) => element.name == toString);
}
```

As far as I know, all enum values are known at compilation time, so perhaps it could be possible to access this list by `E.values`?

Start with the generic findEnum example and the current Foo.values behavior described in the issue. No files, tests, or entry points are named; done would require determining whether type-based access to enum values belongs in the Dart language design and documenting the resulting specification or decision.

Written by the indexing model from the issue text.