Allow recursive extension type about language

[lrhn]

It's not allowed because it's impossible to represent the extension-type erased representation type in the current Dart type system.
Dart would have to add recursive types to the type system before this would be possible.

So this is not an alternative to recursive type aliases, it's basically the same feature, and if we have recursive types in the type system anyway, we might as well allow them in general using recursive type aliases.