Flow should force overriding if a method returns this

While I was brainstorming a solution for #2959 I stumbled upon this issue.

The gist is that flow should force an override of `method` in `B`. Otherwise we can get false positives.
A reproducible case:

```js
class A {
name: string
constructor(name: string) {
this.name = name
}

method(f: (from: string) => string): this {
return new this.constructor(f(this.name))
}
}

class B extends A {
surname: string
constructor(name, surname) {
super(name)
this.surname = surname
}
}

const b = new B('a', 'b')
const b2 = b.method(() => 'b')
// surname is undefined but there's no error
const surname: string = b2.surname
```

If you find it a welcome addition I will try to implement it.