When converting an object to a union containing a sequence, Web IDL
first gets @@iterator to decide whether the sequence arm applies, then
creates the sequence from the iterable using that same method.
The generator instead called sequence_to_idl_value(), which repeated the
@@iterator lookup. Split out create_sequence_from_iterable() and use it
from union sequence/FrozenArray conversion so overridden Symbol.iterator
accessors are only observed once.
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