The return type of an async function must be a supertype of Promise

If a class extends another class where a certain function is sync, with an async function, there will be a warning.

```js
class B {
_transform() {
console.log('sorry not sorry')
}
}

class A extends B {
async _transform() {
console.log('good morning my neighbours')
}
}

class C {
async _transform() {
console.log('google closure compiler rules')
}
}

new A()._transform()
new C()._transform()
```

```sh
t/async.js:8: WARNING - The return type of an async function must be a supertype of Promise
found: undefined
async _transform() {
^^^^^^^^^^^^^^
```