Consider a more modern interface for CompanionWindows

[domenic]

[SecureContext, Exposed=Window]
interface CompanionWindows {
  iterable<CompanionWindow>;
  readonly attribute unsigned long length;
  getter CompanionWindow item(unsigned long index);
  CompanionWindow namedItem(DOMString name);

  Promise<CompanionWindow> open(USVString url, CompanionWindowOpenOptions options = {});
};

This item() / namedItem() style is very old-school API design. Modern APIs generally use something like ObservableArray for indexed access, or maplike<> for named access.

I admit we don't seem to have any modern pattern for both named and indexed access. But is that really important? In particular, I'd be surprised if indexed access was important for many use cases. (And it can be emulated, by maplike.values().drop(i).next().value, if it's really needed. At least once iterator helpers lands.)