react-is breaking change, re AsyncMode

It seems like react-is v16.6 no longer is compatible with React v16.3 - v16.5, in that React v16.3 - v16.5 will use Symbol(react.async_mode), but react-is’s AsyncMode export will instead be Symbol(react.concurrent_mode).

It seems like the async mode symbol needs to remain the same in react-is 16.6+, and the isAsyncMode function would need to check both the AsyncMode and ConcurrentMode symbols.

[gaearon]

If react-is v16.6+ had an AsyncMode symbol export that matched what React 16.3-16.5 did, then I think this wouldn’t be a problem.

I think that sounds okay, we could do it. PR?

[ljharb]

I understand the dilemma. In this case, the functionality hasn’t changed, and I’m not sure why it was renamed - iow i’m not sure how it helps people try it out to have the name change now, when it’s still going to have to change again when it gets promoted to being stable.

The tradeoffs seem to be somewhere between “difficult for people to try it out”, “cause breakage in some amount of downstream tooling”, or “incur a tiny additional bundle cost to maintain back compat”. It’s a tough call, and I’m not asking for a policy change - but in this case, the bundle size increase of #13959 seems pretty close to zero (i can’t imagine it’s more than a handful of bytes), and the benefit is that enzyme (and presumably other downstream tools) will be able to continue using react-is without having to hardcode things.