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

@jakearchibald 

Before I get to the other points you've brought up (I have responses), let me focus on and clarify just this one:

>  signal disinterest in the result, and let a promise react to all observers becoming disinterested.

Let me try to illustrate my question/concern (and perhaps misunderstanding). Assume:

```js
var parent = new Promise(function(resolve){  setTimeout(resolve,100); }),
   child1 = parent.then(function(){ console.log("child1"); }),
   child2 = parent.then(function(){ console.log("child2"); });
```

First, what happens here?

```js
parent.cancel();
```

Do both `"child1"` and `"child2"` still get printed? Or neither?

----

If merely passing `parent` around to various different parts of a system that separately want to observe it means that any one observer (the code that creates `child1`, for example) can unilaterally decide that another part of the system (the code that creates `child2`) is prevented from knowing about what happens with `parent` -- and thus just hangs around waiting in vein -- that's "action at a distance" and is a software design practice that's usually frowned upon. It makes systems harder to reason about and trust. I can follow up with illustrating a `fetch(..)` specific scenario I have in mind, if necessary.

----

Now, what happens if instead:

```js
child1.cancel();
```

Does that mean that `"child2"` does or does not get printed? Same concerns as above.

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