Re: [fetch] Aborting a fetch (#27)

@jakearchibald 

>  that's bad form of the child1 code

The concern is not about what the "child1 code" *should* do, it's about what the "child1 code" *can* do. As promises stand now, "child1 code" cannot affect "child2 code". That's by design, and it's a good thing.

> they can cast to `Promise`

I had a feeling this suggestion was coming soon. So, you're essentially saying:

```js
var p = fetch(..), p_safe = Promise.resolve(p);

// later
child1 = p_safe.then(..)
..
```

Sure, that might work to isolate the capability, since `p_safe` wouldn't have a `cancel(..)`. But then, that two-step you've shown is essentially the same in spirit as:

```js
var controller = fetch(..), p = controller.promise;

// later
child1 = p.then(..)
```

The problem with having "controller" be a `CancelablePromise` and expecting people to understand this nuance and pre-cast with `Promise.resolve(..)` is that most won't understand, and will just pass the cancelable-promise around, until they get bitten, and it won't be entirely clear why.

I'd call that pit-of-failure design.

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