Contextual return types for expression-bodied members

```C#
class C {
	partial A => new partial();
	async B => new async();
}

class partial {}
class async {}
```


The first line compiles; the second one does not.

Why not?
Why is `async` different from `partial`?

http://tryroslyn.azurewebsites.net/#b:master/f:>ilr/MYGwhgzhAEDC0G8BQBIADmATgFwJZhGgEFoBeAPmgDsBTAd2gx3xAAoBKAblUgE8rg0AEJlKtBnwEduAXyRJQkGEzwFEcxVGiTBCGUA=