Alternate fix for "Fix a crash in Suspense with findDOMNode"

[acdlite]

This doesn't rely on checking the tag. When the alternate of a parent is missing, it assumes it's a fragment indirection and moves onto the next parent fiber.

I based my commit on top of @gaearon's PR #15195.

[bvaughn]

Note to self: Assuming we land this, make sure we sync it to the new DevTools package too https://github.com/bvaughn/react-devtools-experimental/blob/07bf8e53c1e6fbf6503bebc170622ab161cf3b8f/src/backend/renderer.js#L1105-L1106

[sizebot]

	Fails
🚫	node` failed.

Log

Error:  { FetchError: invalid json response body at http://react.zpao.com/builds/master/_commits/43b1f74c88d986c88623412be7b1d65a6e271779/results.json reason: Unexpected token < in JSON at position 0
    at /home/circleci/project/node_modules/node-fetch/lib/body.js:48:31
    at process._tickCallback (internal/process/next_tick.js:68:7)
  name: 'FetchError',
  message:
   'invalid json response body at http://react.zpao.com/builds/master/_commits/43b1f74c88d986c88623412be7b1d65a6e271779/results.json reason: Unexpected token < in JSON at position 0',
  type: 'invalid-json' }

Generated by 🚫 dangerJS

[sebmarkbage]

This only deals with one of the paths hitting this case.

If we're traversing backwards in the inverse case, then we'll skip past the extra fragment on one of the nodes. It seems like something else can go wrong in that scenario.

[acdlite]

Oops forgot to run Flow

[sebmarkbage]

What if parentA here is a Suspense boundary with no fragment and its alternate is a Suspense boundary with a fragment?

In that case the children won't be representing the same thing. However, they also will never be equal so maybe it doesn't matter?

[acdlite]

That was my thinking, at least

[acdlite]

I'll squash my commits (but leave @gaearon's separate) and land via command line so @gaearon also gets credit

[bvaughn]

Thanks folks!

Synced to DevTools via bvaughn/react-devtools-experimental@7fbdfd9