Re: [whatwg/dom] Define a 'CancelationController' and 'CancelationSignal' interface. (#434)

@rbuckton thanks, that's an interesting scenario. Let me try to sketch it out in terms of the API we have:
```js
const controller = new AbortController,
      signal = controller.signal
signal.onabort = abortion
fetch("https://test:test/", { signal }).then(..., rejection)
controller.abort()
```
The `fetch()` call directly rejects, with `rejection` being invoked at the end-of-task. `abortion` however is invoked directly after `controller.abort()`, before `rejection`. A solution to that would be to `.then()` the promise generated by the method to which _signal_ is passed and only once that rejects with an "AbortError", flip the state and dispatch the `abort` event.

Over in https://github.com/whatwg/fetch/issues/447 folks wanted to be able to synchronously observe when `abort()` was invoked, though I suppose if that's actually a strong enough use case that could also be solved by the controller.

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