Issue with lint unnecessary_await_in_return

In the code below, await is necessary.
```
import 'dart:async';

Future<num> foo() async => 1;

// The lint unnecessary_await_in_return reports an issue here, but the await is necessary
// as an implicit downcast is necessary.
// We cannot change the code to `Future<int> bar() => foo()`
Future<int> bar() async => await foo();

void main() async => print(await bar());
```
