[act] remove obsolete container element (#16312)

In a previous version of act(), we used a dummy dom element to flush effects. This doesn't need to exist anymore, and this PR removes it. The warning doesn't need to be there either (React will fire a wrong renderer act warning if needed).

packages/react-dom/src/test-utils/ReactTestUtils.js

@@ -4,7 +4,6 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  */
-import type {Thenable} from 'react-reconciler/src/ReactFiberWorkLoop';
 
 import React from 'react';
 import ReactDOM from 'react-dom';
@@ -19,7 +18,6 @@ import {
 import SyntheticEvent from 'events/SyntheticEvent';
 import invariant from 'shared/invariant';
 import lowPriorityWarning from 'shared/lowPriorityWarning';
-import warningWithoutStack from 'shared/warningWithoutStack';
 import {ELEMENT_NODE} from '../shared/HTMLNodeType';
 import * as DOMTopLevelEventTypes from '../events/DOMTopLevelEventTypes';
 import {PLUGIN_EVENT_SYSTEM} from 'events/EventSystemFlags';
@@ -153,12 +151,6 @@ function validateClassInstance(inst, methodName) {
   );
 }
 
-// a plain dom element, lazily initialized, used by act() when flushing effects
-let actContainerElement = null;
-
-// a warning for when you try to use TestUtils.act in a non-browser environment
-let didWarnAboutActInNodejs = false;
-
 /**
  * Utilities for making it easy to test React components.
  *
@@ -395,25 +387,7 @@ const ReactTestUtils = {
   Simulate: null,
   SimulateNative: {},
 
-  act(callback: () => Thenable) {
-    if (actContainerElement === null) {
-      if (__DEV__) {
-        // warn if we're trying to use this in something like node (without jsdom)
-        if (didWarnAboutActInNodejs === false) {
-          didWarnAboutActInNodejs = true;
-          warningWithoutStack(
-            typeof document !== 'undefined' && document !== null,
-            'It looks like you called ReactTestUtils.act(...) in a non-browser environment. ' +
-              "If you're using TestRenderer for your tests, you should call " +
-              'ReactTestRenderer.act(...) instead of ReactTestUtils.act(...).',
-          );
-        }
-      }
-      // now make the stub element
-      actContainerElement = document.createElement('div');
-    }
-    return act(callback);
-  },
+  act,
 };
 
 /**

packages/react-test-renderer/src/__tests__/ReactTestRenderer-test.internal.js

@@ -1022,20 +1022,4 @@ describe('ReactTestRenderer', () => {
     expect(Scheduler).toFlushWithoutYielding();
     ReactTestRenderer.create(<App />);
   });
-
-  // we run this test here because we need a dom-less scope
-  it('warns and throws if you use TestUtils.act instead of TestRenderer.act in node', () => {
-    // we warn when you try to load 2 renderers in the same 'scope'
-    // so as suggested, we call resetModules() to carry on with the test
-    jest.resetModules();
-    const {act} = require('react-dom/test-utils');
-    expect(() => {
-      expect(() => act(() => {})).toThrow('document is not defined');
-    }).toWarnDev(
-      [
-        'It looks like you called ReactTestUtils.act(...) in a non-browser environment',
-      ],
-      {withoutStack: 1},
-    );
-  });
 });