Use testing builds for our own tests

Following up from facebook#17915 where we generated testing builds for `react-dom`, this PR uses those builds for our own tests.

- fixes `ReactFeatureFlags.testing` to use all the default flags
- changes `ReactFeatureFlags.readonly` to return `isTestEnvironment: true` (this might not strictly be needed)
- with jest, mocks `react-dom` with the testing version, both for source and builds
- uses `ReactDOM.act` in one of these tests to verify it actually works

Author: threepointone

packages/react-dom/package.json

@@ -30,6 +30,7 @@
     "README.md",
     "build-info.json",
     "index.js",
+    "testing.js",
     "profiling.js",
     "server.js",
     "server.browser.js",

packages/react-dom/src/__tests__/ReactDOMSuspensePlaceholder-test.js

@@ -13,7 +13,6 @@ let React;
 let ReactDOM;
 let Suspense;
 let ReactCache;
-let ReactTestUtils;
 let Scheduler;
 let TextResource;
 let act;
@@ -26,9 +25,8 @@ describe('ReactDOMSuspensePlaceholder', () => {
     React = require('react');
     ReactDOM = require('react-dom');
     ReactCache = require('react-cache');
-    ReactTestUtils = require('react-dom/test-utils');
     Scheduler = require('scheduler');
-    act = ReactTestUtils.act;
+    act = ReactDOM.act;
     Suspense = React.Suspense;
     container = document.createElement('div');
     document.body.appendChild(container);

packages/shared/forks/ReactFeatureFlags.readonly.js

@@ -9,4 +9,4 @@
 // It lets us determine whether we're running in Fire mode without making tests internal.
 const ReactFeatureFlags = require('../ReactFeatureFlags');
 // Forbid writes because this wouldn't work with bundle tests.
-module.exports = Object.freeze({...ReactFeatureFlags});
+module.exports = Object.freeze({...ReactFeatureFlags, isTestEnvironment: true});

packages/shared/forks/ReactFeatureFlags.testing.js

@@ -7,25 +7,25 @@
  * @flow
  */
 
-import invariant from 'shared/invariant';
+// keep in sync with ReactFeatureFlags.js
+// only isTestEnvironment is different, and true
 
 import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
 import typeof * as PersistentFeatureFlagsType from './ReactFeatureFlags.persistent';
 
-export const debugRenderPhaseSideEffectsForStrictMode = false;
 export const enableUserTimingAPI = __DEV__;
+export const debugRenderPhaseSideEffectsForStrictMode = __DEV__;
+export const replayFailedUnitOfWorkWithInvokeGuardedCallback = __DEV__;
 export const warnAboutDeprecatedLifecycles = true;
-export const replayFailedUnitOfWorkWithInvokeGuardedCallback = false;
 export const enableProfilerTimer = __PROFILE__;
 export const enableSchedulerTracing = __PROFILE__;
-export const enableSuspenseServerRenderer = false;
-export const enableSelectiveHydration = false;
-export const enableChunksAPI = false;
+export const enableSuspenseServerRenderer = __EXPERIMENTAL__;
+export const enableSelectiveHydration = __EXPERIMENTAL__;
+export const enableChunksAPI = __EXPERIMENTAL__;
+export const enableSchedulerDebugging = false;
 export const disableJavaScriptURLs = false;
-export const disableInputAttributeSyncing = false;
 export const exposeConcurrentModeAPIs = __EXPERIMENTAL__;
 export const warnAboutShorthandPropertyCollision = false;
-export const enableSchedulerDebugging = false;
 export const enableDeprecatedFlareAPI = false;
 export const enableFundamentalAPI = false;
 export const enableScopeAPI = false;
@@ -34,23 +34,24 @@ export const warnAboutUnmockedScheduler = false;
 export const flushSuspenseFallbacksInTests = true;
 export const enableSuspenseCallback = false;
 export const warnAboutDefaultPropsOnFunctionComponents = false;
-export const warnAboutStringRefs = false;
-export const disableLegacyContext = false;
 export const disableSchedulerTimeoutBasedOnReactExpirationTime = false;
 export const enableTrainModelFix = true;
 export const enableTrustedTypesIntegration = false;
 export const enableNativeTargetAsInstance = false;
+export const deferPassiveEffectCleanupDuringUnmount = false;
+export const disableInputAttributeSyncing = false;
+export const warnAboutStringRefs = false;
+export const disableLegacyContext = false;
 export const disableCreateFactory = false;
 export const disableTextareaChildren = false;
 export const disableUnstableRenderSubtreeIntoContainer = false;
 export const warnUnstableRenderSubtreeIntoContainer = false;
 export const disableUnstableCreatePortal = false;
-export const deferPassiveEffectCleanupDuringUnmount = false;
 export const isTestEnvironment = true;
 
 // Only used in www builds.
 export function addUserTimingListener() {
-  invariant(false, 'Not implemented.');
+  throw new Error('Not implemented.');
 }
 
 // Flow magic to verify the exports of this file match the original version.

scripts/jest/config.base.js

@@ -4,6 +4,9 @@ module.exports = {
   haste: {
     hasteImplModulePath: require.resolve('./noHaste.js'),
   },
+  moduleNameMapper: {
+    '^shared/ReactFeatureFlags': `<rootDir>/packages/shared/forks/ReactFeatureFlags.testing`,
+  },
   modulePathIgnorePatterns: [
     '<rootDir>/scripts/rollup/shims/',
     '<rootDir>/scripts/bench/',

scripts/jest/setupHostConfigs.js

@@ -177,6 +177,8 @@ inlinedHostConfigs.forEach(rendererInfo => {
   }
 });
 
+jest.mock('react-dom', () => require.requireActual('react-dom/testing'));
+
 // Make it possible to import this module inside
 // the React package itself.
 jest.mock('shared/ReactSharedInternals', () =>

scripts/jest/setupTests.build.js

@@ -1,5 +1,7 @@
 'use strict';
 
+jest.mock('react-dom', () => require.requireActual(`react-dom/testing`));
+
 jest.mock('scheduler', () => require.requireActual('scheduler/unstable_mock'));
 jest.mock('scheduler/src/SchedulerHostConfig', () =>
   require.requireActual('scheduler/src/forks/SchedulerHostConfig.mock.js')