Because the JS community's polyfilling infrastructure sucks and we'll
have to fix it for them before we require this.
JSX spread uses React.__spread
(which might get special behavior for key/ref, not sure yet)
This never uses the native implementation and throws for prototype chains.
Once the native implementations are faster, we'll start using them.
This makes it easier to contribute without having to learn a bunch of
slightly different helpers and remember their slightly different
signatures and semantics.
We'll probably start using ES7 spread properties instead of merge in the
future when at least one more transpiler supports it.
Sebastian Markbage
Paul O’Shannessy
Miorel Palii
Kunal Mehta