dillon1000/react

Resolve refs in the order of the children (#7101)

* Resolve refs in the order of the children

React makes no guarantees about ref resolution order. Unfortunately, some of the internal Facebook component APIs (specifically, layer dialogs) currently depend on the ref resolution order. Specifically, the assumption is that if the layer dialog is placed as a last child, by the time it mounts or updates, the refs to any previously declared elements have been resolved.

With the current `ReactMultiChild`, this is *usually* the case but not always. Both initial mount and an update of all components satisfy this assumption: by the time a child mounts or updates, the previous children’s refs have been resolved. The one scenario where it isn’t true is when **a new child is mounted during an update**.

In this case, the `mountComponent()` call used to be delayed until `ReactMultiChild` processes the queue. However, this is inconsistent with how updates normally work: unlike mounting, updating and unmounting happens inside `ReactChildReconciler.updateChildren()` loop.

This PR changes the `mountComponent()` to be performed inside `ReactChildReconciler`, just like `receiveComponent()` and `unmountComponent()`, and thus ensures that `attachRef()` calls are enqueued in the order the children were processed, so by the time the next child flushes, the refs of the previous children have been resolved.

This is not ideal and will probably be broken by incremental reconciler in the future. However, since we are trying to get rid of mixins in the internal codebase, and layered components are one of the biggest blockers to that, it’s lesser evil to temporarily make ref resolution order more strict until we have time to fix up the layer APIs to not rely on it, and are able to relax it again (which would be a breaking change).

* Use array instead of object to avoid lookups