Unexpected unnecessary_cast when cast is required for argument type

[natebosch]

String takeNullable(Iterable<String>? i) => takeIterable(i ?? []); // Object can't be assigned to Iterable<Object>
String takeIterable(Iterable<Object> i) => '';

String takeNullable(Iterable<String>? i) => takeIterable((i ?? []) as Iterable<Object>); // unnecessary_cast
String takeIterable(Iterable<Object> i) => '';

This cast is necessary. Using a different pattern requires changing from => to {return}.

[natebosch]

I think the cast should become unnecessary if we implemented dart-lang/language#2259