Cross-realm well-known symbols

[anilanar]

WellKnownSymbolsStore is not shared across realms, but Symbol.for is shared:

https://jakearchibald.com/2017/arrays-symbols-realms/

Reproduction:

1. Load Symbol.observable polyfill in a modern/recent browser.
2. Create same domain iframe and load another copy of Symbol.observable polyfill in the iframe.
3. Compare Symbol.observable against Symbol.observable from the iframe, they won't be equal.

On the other hand, window.Symbol.for('observable') === iframe.window.Symbol.for('observable') so Symbol.for works across realms.

[zloirock]

Thanks for the issue. I thought about it. But someone could use Symbol.for('observable') for anything else and it will unexpectedly make objects with this property observable. However, sure, it could be solved by using something like prefixes. Also, it will not work with polyfilled Symbol - the proper cross-realm Symbol.for behavior can't be polyfilled, so we could forget about, for example, IE11 - that was the main reason why it's still not added.