[38179] added support for the rainbow flag to core, but platform compatibility is a bit strange, because the flag was defined out of sync with the usual Unicode release cycle. For example Android N supports all Unicode 9.0 emoji, but doesn't support the rainbow flag thus the rainbow flag isn't rendered.
The Twemoji loader already has an exception for flags - it'll load if the browser supports everything but flags, and then only replace flag emoji, leaving everything else as native rendering. To provide a seamless experience for all flags the loader now uses the rainbow flag to test flag support.
Merge of [38194] to the 4.6 branch.
Props pento.
See #37543.
See #37566.
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