The union-returning flatMap helper was interned by element and result
types alone, so two callbacks with one union result type, one with an
empty `never[]` arm and one with a populated array arm, shared the first
helper and the second dropped its elements. The key now includes the
per-arm plan. Array arms copy their elements as stored, a present
undefined included, so an array arm's element type must be the result's,
and a unit-typed result (`undefined`) appends itself.
Chris Tate
4e06e72b75