Add SuspenseList to react-is (#20874)

This commit also adds explicit index.stable and index.experimental forks to the react-is package so that we can avoid exporting references to SuspenseList in a stable release.

packages/react-is/index.experimental.js

@@ -0,0 +1,41 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+'use strict';
+
+export {
+  isValidElementType,
+  typeOf,
+  ContextConsumer,
+  ContextProvider,
+  Element,
+  ForwardRef,
+  Fragment,
+  Lazy,
+  Memo,
+  Portal,
+  Profiler,
+  StrictMode,
+  Suspense,
+  unstable_SuspenseList,
+  isAsyncMode,
+  isConcurrentMode,
+  isContextConsumer,
+  isContextProvider,
+  isElement,
+  isForwardRef,
+  isFragment,
+  isLazy,
+  isMemo,
+  isPortal,
+  isProfiler,
+  isStrictMode,
+  isSuspense,
+  unstable_isSuspenseList,
+} from './src/ReactIs';

packages/react-is/index.stable.js

@@ -0,0 +1,39 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+'use strict';
+
+export {
+  isValidElementType,
+  typeOf,
+  ContextConsumer,
+  ContextProvider,
+  Element,
+  ForwardRef,
+  Fragment,
+  Lazy,
+  Memo,
+  Portal,
+  Profiler,
+  StrictMode,
+  Suspense,
+  isAsyncMode,
+  isConcurrentMode,
+  isContextConsumer,
+  isContextProvider,
+  isElement,
+  isForwardRef,
+  isFragment,
+  isLazy,
+  isMemo,
+  isPortal,
+  isProfiler,
+  isStrictMode,
+  isSuspense,
+} from './src/ReactIs';

packages/react-is/src/ReactIs.js

@@ -72,6 +72,7 @@ export const Portal = REACT_PORTAL_TYPE;
 export const Profiler = REACT_PROFILER_TYPE;
 export const StrictMode = REACT_STRICT_MODE_TYPE;
 export const Suspense = REACT_SUSPENSE_TYPE;
+export const unstable_SuspenseList = REACT_SUSPENSE_LIST_TYPE;
 
 export {isValidElementType};
 
@@ -142,3 +143,6 @@ export function isStrictMode(object: any) {
 export function isSuspense(object: any) {
   return typeOf(object) === REACT_SUSPENSE_TYPE;
 }
+export function unstable_isSuspenseList(object: any) {
+  return typeOf(object) === REACT_SUSPENSE_LIST_TYPE;
+}

packages/react-is/src/__tests__/ReactIs-test.js

@@ -186,6 +186,24 @@ describe('ReactIs', () => {
     expect(ReactIs.isSuspense(<div />)).toBe(false);
   });
 
+  // @gate experimental
+  it('should identify suspense list', () => {
+    expect(ReactIs.isValidElementType(React.unstable_SuspenseList)).toBe(true);
+    expect(ReactIs.typeOf(<React.unstable_SuspenseList />)).toBe(
+      ReactIs.unstable_SuspenseList,
+    );
+    expect(
+      ReactIs.unstable_isSuspenseList(<React.unstable_SuspenseList />),
+    ).toBe(true);
+    expect(
+      ReactIs.unstable_isSuspenseList({type: ReactIs.unstable_SuspenseList}),
+    ).toBe(false);
+    expect(ReactIs.unstable_isSuspenseList('React.unstable_SuspenseList')).toBe(
+      false,
+    );
+    expect(ReactIs.unstable_isSuspenseList(<div />)).toBe(false);
+  });
+
   it('should identify profile root', () => {
     expect(ReactIs.isValidElementType(React.Profiler)).toBe(true);
     expect(