Add test to ensure setUpDefaultReactNativeEnvironment does not access feature flags

packages/react-native/src/private/featureflags/ReactNativeFeatureFlagsBase.js

@@ -18,6 +18,10 @@ import NativeReactNativeFeatureFlags from './specs/NativeReactNativeFeatureFlags
 const accessedFeatureFlags: Set<string> = new Set();
 let overrides: ?ReactNativeFeatureFlagsJsOnlyOverrides;
 
+// This is a list of functions to clear the cached value for each feature flag
+// getter. This is only used in development.
+const clearCachedValuesFns: Array<() => void> = [];
+
 export type Getter<T> = () => T;
 
 // This defines the types for the overrides object, whose methods also receive
@@ -33,6 +37,12 @@ function createGetter<T: boolean | number | string>(
 ): Getter<T> {
   let cachedValue: ?T;
 
+  if (__DEV__) {
+    clearCachedValuesFns.push(() => {
+      cachedValue = undefined;
+    });
+  }
+
   return () => {
     if (cachedValue == null) {
       cachedValue = customValueGetter() ?? defaultValue;
@@ -115,3 +125,12 @@ function maybeLogUnavailableNativeModuleError(configName: string): void {
     );
   }
 }
+
+export function dangerouslyResetForTesting(): void {
+  if (__DEV__) {
+    overrides = null;
+    accessedFeatureFlags.clear();
+    reportedConfigNames.clear();
+    clearCachedValuesFns.forEach(fn => fn());
+  }
+}

packages/react-native/src/private/setup/__tests__/setUpDefaultReactNativeEnvironment-FeatureFlags-itest.js

@@ -0,0 +1,28 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ */
+
+import * as ReactNativeFeatureFlags from '../../featureflags/ReactNativeFeatureFlags';
+import * as ReactNativeFeatureFlagsBase from '../../featureflags/ReactNativeFeatureFlagsBase';
+import setUpDefaultReactNativeEnvironment from 'react-native/src/private/setup/setUpDefaultReactNativeEnvironment';
+
+describe('setUpReactNativeEnvironment', () => {
+  it('should not read any feature flags', () => {
+    ReactNativeFeatureFlagsBase.dangerouslyResetForTesting();
+
+    setUpDefaultReactNativeEnvironment(false);
+
+    expect(() => {
+      // If any feature flags were read, this call would fail.
+      ReactNativeFeatureFlags.override({
+        jsOnlyTestFlag: () => true,
+      });
+    }).not.toThrow();
+  });
+});