CancelableOperation and CancelableCompleter should cancel/kill delayed Futures

When starting a Future which contains an `await` somewhere in the code, be it a delay or just a network request waiting for a response, it is impossible to stop that Future, not even with `CancelableOperation` or `CancelableCompleter`.

If the Future is running in the main thread without any `await`, it can't be stopped anyway, because the cancel operation is only invoked when the Future is done.

```dart
import 'dart:async';
import 'package:async/async.dart';

void main() {
Future foo() async {
print('running...');
await Future.delayed(Duration(seconds: 5));
print('done');
}

final CancelableCompleter completer = CancelableCompleter(onCancel: () => print('onCancel'));
completer.complete(foo());
completer.operation.cancel().then((dynamic _) => print('cancelled'));
}
```

Output:
```
running...
onCancel
cancelled
done // appears after 5 seconds
```

Start by reviewing the Dart example and the stated behavior of CancelableOperation and CancelableCompleter around Future.delayed and awaited network requests. The issue does not name files, tests, or an entry point, and completion criteria for changing Future cancellation semantics are not specified.

Written by the indexing model from the issue text.