Relax return type for setTimeout and setInterval to fix build
Relax return type for setTimeout and setInterval to fix build

Summary:
For callbacks who's return values are ignored, we often type
the return type as `mixed`. This allows using async callbacks, which
return `Promise`. Furthermore, sometimes people write code like this:

```
class Foo {
  setBar(bar) {
    this.bar = bar;
    return this;
  }
}

const foo = new Foo();
setTimeout(() => foo.setBar(100), 1000);
```

Since we're using the `args => expr` arrow function, the return value is
the value of `expr`. Sure, we could switch to `args => { expr }`, but
the curly-less version is pretty idiomatic.

Reviewed By: dalongi, samwgoldman

Differential Revision: D6592691

fbshipit-source-id: dfc6828000e5f7c4d951eb9bf0e33f2c205855a5

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