Setup Wave 2 of Feature Flags for React Native (#28990)

## Summary

Sets up dynamic feature flags for `disableStringRefs`, `enableFastJSX`,
and `enableRefAsProp` in React Native (at Meta).

## How did you test this change?

```
$ yarn test
$ yarn flow fabric
```

DiffTrain build for commit 9b13002.

compiled-rn/facebook-fbsource/xplat/js/RKJSModules/vendor/react/react-test-renderer/cjs/ReactTestRenderer-dev.js

@@ -7,7 +7,7 @@
  * @noflow
  * @nolint
  * @preventMunge
- * @generated SignedSource<<3cde5a560aeab39ea15c34b2e5562835>>
+ * @generated SignedSource<<b49fe2332655e16fd76d120df3e46271>>
  */
 
 'use strict';
@@ -5057,21 +5057,6 @@ function shallowEqual(objA, objB) {
 
 var current = null;
 var isRendering = false;
-function getCurrentFiberOwnerNameInDevOrNull() {
-  {
-    if (current === null) {
-      return null;
-    }
-
-    var owner = current._debugOwner;
-
-    if (owner != null) {
-      return getComponentNameFromOwner(owner);
-    }
-  }
-
-  return null;
-}
 
 function getCurrentFiberStackInDev() {
   {
@@ -5665,8 +5650,11 @@ function coerceRef(returnFiber, current, workInProgress, element) {
   var ref;
 
   {
-    // Old behavior.
-    ref = element.ref;
+    // TODO: This is a temporary, intermediate step. When enableRefAsProp is on,
+    // we should resolve the `ref` prop during the begin phase of the component
+    // it's attached to (HostComponent, ClassComponent, etc).
+    var refProp = element.props.ref;
+    ref = refProp !== undefined ? refProp : null;
   } // TODO: If enableRefAsProp is on, we shouldn't use the `ref` field. We
   // should always read the ref from the prop.
 
@@ -11472,6 +11460,19 @@ function resolveClassComponentProps(Component, baseProps, // Only resolve defaul
 alreadyResolvedDefaultProps) {
   var newProps = baseProps;
 
+  {
+    // Remove ref from the props object, if it exists.
+    if ('ref' in baseProps) {
+      newProps = {};
+
+      for (var propName in baseProps) {
+        if (propName !== 'ref') {
+          newProps[propName] = baseProps[propName];
+        }
+      }
+    }
+  } // Resolve default props.
+
 
   var defaultProps = Component.defaultProps;
 
@@ -12076,7 +12077,6 @@ var didReceiveUpdate = false;
 var didWarnAboutBadClass;
 var didWarnAboutContextTypeOnFunctionComponent;
 var didWarnAboutGetDerivedStateOnFunctionComponent;
-var didWarnAboutFunctionRefs;
 var didWarnAboutReassigningProps;
 var didWarnAboutRevealOrder;
 var didWarnAboutTailOptions;
@@ -12086,7 +12086,6 @@ var didWarnAboutDefaultPropsOnFunctionComponent;
   didWarnAboutBadClass = {};
   didWarnAboutContextTypeOnFunctionComponent = {};
   didWarnAboutGetDerivedStateOnFunctionComponent = {};
-  didWarnAboutFunctionRefs = {};
   didWarnAboutReassigningProps = false;
   didWarnAboutRevealOrder = {};
   didWarnAboutTailOptions = {};
@@ -12135,7 +12134,21 @@ function updateForwardRef(current, workInProgress, Component, nextProps, renderL
   var ref = workInProgress.ref;
   var propsWithoutRef;
 
-  {
+  if ('ref' in nextProps) {
+    // `ref` is just a prop now, but `forwardRef` expects it to not appear in
+    // the props object. This used to happen in the JSX runtime, but now we do
+    // it here.
+    propsWithoutRef = {};
+
+    for (var key in nextProps) {
+      // Since `ref` should only appear in props via the JSX transform, we can
+      // assume that this is a plain object. So we don't need a
+      // hasOwnProperty check.
+      if (key !== 'ref') {
+        propsWithoutRef[key] = nextProps[key];
+      }
+    }
+  } else {
     propsWithoutRef = nextProps;
   } // The rest is a fork of updateFunctionComponent
 
@@ -12567,19 +12580,6 @@ function markRef(current, workInProgress) {
     }
 
     if (current === null || current.ref !== ref) {
-      if (current !== null) {
-        var oldRef = current.ref;
-        var newRef = ref;
-
-        if (typeof oldRef === 'function' && typeof newRef === 'function' && typeof oldRef.__stringRef === 'string' && oldRef.__stringRef === newRef.__stringRef && oldRef.__stringRefType === newRef.__stringRefType && oldRef.__stringRefOwner === newRef.__stringRefOwner) {
-          // Although this is a different callback, it represents the same
-          // string ref. To avoid breaking old Meta code that relies on string
-          // refs only being attached once, reuse the old ref. This will
-          // prevent us from detaching and reattaching the ref on each update.
-          workInProgress.ref = oldRef;
-          return;
-        }
-      } // Schedule a Ref effect
 
 
       workInProgress.flags |= Ref | RefStatic;
@@ -13064,24 +13064,6 @@ function validateFunctionComponentInDev(workInProgress, Component) {
       }
     }
 
-    if (workInProgress.ref !== null) {
-      var info = '';
-      var componentName = getComponentNameFromType(Component) || 'Unknown';
-      var ownerName = getCurrentFiberOwnerNameInDevOrNull();
-
-      if (ownerName) {
-        info += '\n\nCheck the render method of `' + ownerName + '`.';
-      }
-
-      var warningKey = componentName + '|' + (ownerName || '');
-
-      if (!didWarnAboutFunctionRefs[warningKey]) {
-        didWarnAboutFunctionRefs[warningKey] = true;
-
-        error('Function components cannot be given refs. ' + 'Attempts to access this ref will fail. ' + 'Did you mean to use React.forwardRef()?%s', info);
-      }
-    }
-
     if (Component.defaultProps !== undefined) {
       var _componentName = getComponentNameFromType(Component) || 'Unknown';
 
@@ -17259,7 +17241,9 @@ function commitAttachRef(finishedWork) {
       {
         // TODO: We should move these warnings to happen during the render
         // phase (markRef).
-        if (!ref.hasOwnProperty('current')) {
+        if (typeof ref === 'string') {
+          error('String refs are no longer supported.');
+        } else if (!ref.hasOwnProperty('current')) {
           error('Unexpected ref object provided for %s. ' + 'Use either a ref-setter function or React.createRef().', getComponentNameFromFiber(finishedWork));
         }
       } // $FlowFixMe[incompatible-use] unable to narrow type to the non-function case
@@ -23311,7 +23295,7 @@ identifierPrefix, onUncaughtError, onCaughtError, onRecoverableError, transition
   return root;
 }
 
-var ReactVersion = '19.0.0-beta-860ea752';
+var ReactVersion = '19.0.0-beta-7be9cfc9';
 
 /*
  * The `'' + value` pattern (used in perf-sensitive code) throws for Symbol