Re: [ServiceWorker] Cancelling HTTP fetch (#592)

I have one more proposal for canceling ```fetch```. Let's image what ```fetch``` should not return ```Promise``` (or any subclasses of it if it's hard for now). but some ```FetchObject``` with similar signature as ```Promise```:
```javascript
FetchObject = {
  // promise-like
  then: function() {},
  catch: function() {},
 
  // cancel() method not related to promise at all
  cancel: function() {}
};
```

```then()``` and ```catch()``` will pretend as FetchObject is promise (like third-party promises), but will return native ```Promise```. In the meantime, ```cancel()``` will be related to ```fetch``` itself and its execution e.g. aborting request and preventing promise to be settled (or rejecting it, I cannot remember which action you already choose for it) if it was not already, and closing the stream (sorry I am not familiar with steams terminology here) if promise was fullfilled/stream was used.

In general, usage would look like this:
```javascript
var reqSomething = fetch('...');
var asyncDoSomethig = ...;

var onceDone = Promise.all([
  reqSomething.then(function handleStream() { ... });,
  asyncRenderSomething
]);

onceDone.then(function(args) {
    // apply some stuff
  });

onEscPressed(function() {
   reqSomething.cancel();
});
```

So here ```onceDoen.then``` will never called if esc would be pressed before fetch is completely handled.

Few other things:
As someone pointed out, canceling is not always required it will be used once needed, for example this will not work, because ```reqSomething``` will not be ```FetchObject```, but regular ```Promise```:
```javascript
var reqSomething = fetch('...').reqSomething.then(function handleStream() { ... });
reqSomething.cancel();
```

So once someone needed to cancel fetch, then they may store origin ```FetchObject``` to do so. And, of course, it would be to have ```cancel``` event for ```FetchObject```, so it probably should be implement ```EventTarget``` too.

This was seems not ideal, but other ways are not better. Thanks.



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https://github.com/slightlyoff/ServiceWorker/issues/592#issuecomment-85461838