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

@NekR that's already possible with the shipped API

```js
fetch(url).then(response => {
  if (response.headers.get('Content-Type') !== 'application/json') {
    response.body.cancel();
  }
});
```

We can already abort the response, it's the request we can't abort. The only case this is a problem is when the request is particularly large, say you're uploading a large file.

It's trivial to do what you're suggesting whilst still returning a promise. `fetch()` could return a subclass that has an abort method that terminates the request or in-progress response stream. The [`@@species`](https://people.mozilla.org/~jorendorff/es6-draft.html#sec-get-promise-@@species) of that subclass would be `Promise`, so calls to `.then` would return a regular promise, and there's no chain to worry about.

The question is whether there's a benefit in the `@@species` also being abortable.

```js
// If @@species is a regular promise:
var fetchPromise = fetch(url);
var jsonPromise = fetchPromise.then(r => r.json());
// To abort the request & response:
fetchPromise.abort();

// If @@species is abortable:
var jsonPromise = fetch(url).then(r => r.json());
// To abort the request & response:
jsonPromise.abort();
```

---
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https://github.com/whatwg/fetch/issues/27#issuecomment-86981339