Should we have generic getters?

Given that we plan to allow `name` as an expression, do we then lift the restriction that getters cannot be generic?

This could be convenient, e.g., for the expression of "cast" getters:

```dart
extension AsExtension on Object? {
X get as => this as X;
X? get asOrNull {
var self = this;
return self is X ? self : null;
}
}

extension AsSubtypeExtension on X {
Y get asSubtype => this as Y;
}

void main() {
num? n = 1 as dynamic;
n.as.isEven;
n.asSubtype.isEven;
n.asOrNull?.isEven;
}
```

No files, tests, or entry points are named. Read the proposal and examples in issue #1622 first, then determine whether generic getters should be permitted and capture the resulting language decision or specification change as done.

Written by the indexing model from the issue text.