Fix Missing key Validation in React.Children (#29675)

## Summary

In #29088, the validation logic
for `React.Children` inspected whether `mappedChild` — the return value
of the map callback — has a valid `key`. However, this deviates from
existing behavior which only warns if the original `child` is missing a
required `key`.

This fixes false positive `key` validation warnings when using
`React.Children`, by validating the original `child` instead of
`mappedChild`.

This is a more general fix that expands upon my previous fix in
#29662.

## How did you test this change?

```
$ yarn test ReactChildren-test.js
```

DiffTrain build for [8fd963a](8fd963a)

Author: yungsters

compiled/facebook-www/REVISION

@@ -1 +1 @@
-aa3d6c0840357eb469df9bd1c20b201197ce3bdc
+8fd963a1e5ec89459cac27fb1d9ad193a0604110

compiled/facebook-www/REVISION_TRANSFORMS

@@ -1 +1 @@
-aa3d6c0840357eb469df9bd1c20b201197ce3bdc
+8fd963a1e5ec89459cac27fb1d9ad193a0604110

compiled/facebook-www/React-dev.classic.js

@@ -22,7 +22,7 @@ if (
 ) {
   __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStart(new Error());
 }
-var ReactVersion = '19.0.0-www-classic-aa3d6c0840-20240530';
+var ReactVersion = '19.0.0-www-classic-8fd963a1e5-20240530';
 
 // Re-export dynamic flags from the www version.
 var dynamicFeatureFlags = require('ReactFeatureFlags');
@@ -2330,11 +2330,16 @@ function mapIntoArray(children, array, escapedPrefix, nameSoFar, callback) {
         ) + '/' : '') + childKey);
 
         {
-          if (nameSoFar !== '' && mappedChild.key == null) {
-            // We need to validate that this child should have had a key before assigning it one.
-            if (!newChild._store.validated) {
-              // We mark this child as having failed validation but we let the actual renderer
-              // print the warning later.
+          // If `child` was an element without a `key`, we need to validate if
+          // it should have had a `key`, before assigning one to `mappedChild`.
+          // $FlowFixMe[incompatible-type] Flow incorrectly thinks React.Portal doesn't have a key
+          if (nameSoFar !== '' && _child != null && isValidElement(_child) && _child.key == null) {
+            // We check truthiness of `child._store.validated` instead of being
+            // inequal to `1` to provide a bit of backward compatibility for any
+            // libraries (like `fbt`) which may be hacking this property.
+            if (_child._store && !_child._store.validated) {
+              // Mark this child as having failed validation, but let the actual
+              // renderer print the warning later.
               newChild._store.validated = 2;
             }
           }

compiled/facebook-www/React-dev.modern.js

@@ -22,7 +22,7 @@ if (
 ) {
   __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStart(new Error());
 }
-var ReactVersion = '19.0.0-www-modern-aa3d6c0840-20240530';
+var ReactVersion = '19.0.0-www-modern-8fd963a1e5-20240530';
 
 // Re-export dynamic flags from the www version.
 var dynamicFeatureFlags = require('ReactFeatureFlags');
@@ -2333,11 +2333,16 @@ function mapIntoArray(children, array, escapedPrefix, nameSoFar, callback) {
         ) + '/' : '') + childKey);
 
         {
-          if (nameSoFar !== '' && mappedChild.key == null) {
-            // We need to validate that this child should have had a key before assigning it one.
-            if (!newChild._store.validated) {
-              // We mark this child as having failed validation but we let the actual renderer
-              // print the warning later.
+          // If `child` was an element without a `key`, we need to validate if
+          // it should have had a `key`, before assigning one to `mappedChild`.
+          // $FlowFixMe[incompatible-type] Flow incorrectly thinks React.Portal doesn't have a key
+          if (nameSoFar !== '' && _child != null && isValidElement(_child) && _child.key == null) {
+            // We check truthiness of `child._store.validated` instead of being
+            // inequal to `1` to provide a bit of backward compatibility for any
+            // libraries (like `fbt`) which may be hacking this property.
+            if (_child._store && !_child._store.validated) {
+              // Mark this child as having failed validation, but let the actual
+              // renderer print the warning later.
               newChild._store.validated = 2;
             }
           }

compiled/facebook-www/React-prod.classic.js

@@ -684,4 +684,4 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.0.0-www-classic-aa3d6c0840-20240530";
+exports.version = "19.0.0-www-classic-8fd963a1e5-20240530";

compiled/facebook-www/React-prod.modern.js

@@ -684,4 +684,4 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.0.0-www-modern-aa3d6c0840-20240530";
+exports.version = "19.0.0-www-modern-8fd963a1e5-20240530";

compiled/facebook-www/React-profiling.classic.js

@@ -688,7 +688,7 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.0.0-www-classic-aa3d6c0840-20240530";
+exports.version = "19.0.0-www-classic-8fd963a1e5-20240530";
 "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
   "function" ===
     typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

compiled/facebook-www/React-profiling.modern.js

@@ -688,7 +688,7 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactSharedInternals.H.useTransition();
 };
-exports.version = "19.0.0-www-modern-aa3d6c0840-20240530";
+exports.version = "19.0.0-www-modern-8fd963a1e5-20240530";
 "undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ &&
   "function" ===
     typeof __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStop &&

compiled/facebook-www/ReactART-dev.classic.js

@@ -60,7 +60,7 @@ function _assertThisInitialized(self) {
   return self;
 }
 
-var ReactVersion = '19.0.0-www-classic-aa3d6c0840-20240530';
+var ReactVersion = '19.0.0-www-classic-8fd963a1e5-20240530';
 
 var LegacyRoot = 0;
 var ConcurrentRoot = 1;

compiled/facebook-www/ReactART-dev.modern.js

@@ -60,7 +60,7 @@ function _assertThisInitialized(self) {
   return self;
 }
 
-var ReactVersion = '19.0.0-www-modern-aa3d6c0840-20240530';
+var ReactVersion = '19.0.0-www-modern-8fd963a1e5-20240530';
 
 var LegacyRoot = 0;
 var ConcurrentRoot = 1;