Replace old Fiber Scheduler with new one (#15387)

The new Fiber Scheduler has been running in Facebook for several days
without issues. Let's switch to it.

package.json

@@ -102,7 +102,6 @@
     "test": "cross-env NODE_ENV=development jest --config ./scripts/jest/config.source.js",
     "test-persistent": "cross-env NODE_ENV=development jest --config ./scripts/jest/config.source-persistent.js",
     "test-fire": "cross-env NODE_ENV=development jest --config ./scripts/jest/config.source-fire.js",
-    "test-new-scheduler": "cross-env NODE_ENV=development jest --config ./scripts/jest/config.source-new-scheduler.js",
     "test-prod": "cross-env NODE_ENV=production jest --config ./scripts/jest/config.source.js",
     "test-fire-prod": "cross-env NODE_ENV=production jest --config ./scripts/jest/config.source-fire.js",
     "test-prod-build": "yarn test-build-prod",

packages/react-dom/src/__tests__/ReactDOMHooks-test.internal.js → packages/react-dom/src/__tests__/ReactDOMHooks-test.js

@@ -9,8 +9,6 @@
 
 'use strict';
 
-let ReactFeatureFlags;
-let enableNewScheduler;
 let React;
 let ReactDOM;
 let Scheduler;
@@ -21,8 +19,6 @@ describe('ReactDOMHooks', () => {
   beforeEach(() => {
     jest.resetModules();
 
-    ReactFeatureFlags = require('shared/ReactFeatureFlags');
-    enableNewScheduler = ReactFeatureFlags.enableNewScheduler;
     React = require('react');
     ReactDOM = require('react-dom');
     Scheduler = require('scheduler');
@@ -101,30 +97,15 @@ describe('ReactDOMHooks', () => {
     }
 
     ReactDOM.render(<Foo />, container);
-
-    if (enableNewScheduler) {
-      // The old behavior was accidental; in the new scheduler, flushing passive
-      // effects also flushes synchronous work, even inside batchedUpdates.
-      ReactDOM.unstable_batchedUpdates(() => {
-        _set(0); // Forces the effect to be flushed
-        expect(otherContainer.textContent).toBe('A');
-        ReactDOM.render(<B />, otherContainer);
-        expect(otherContainer.textContent).toBe('A');
-      });
-      expect(otherContainer.textContent).toBe('B');
-      expect(calledA).toBe(true);
-      expect(calledB).toBe(true);
-    } else {
-      ReactDOM.unstable_batchedUpdates(() => {
-        _set(0); // Forces the effect to be flushed
-        expect(otherContainer.textContent).toBe('');
-        ReactDOM.render(<B />, otherContainer);
-        expect(otherContainer.textContent).toBe('');
-      });
-      expect(otherContainer.textContent).toBe('B');
-      expect(calledA).toBe(false); // It was in a batch
-      expect(calledB).toBe(true);
-    }
+    ReactDOM.unstable_batchedUpdates(() => {
+      _set(0); // Forces the effect to be flushed
+      expect(otherContainer.textContent).toBe('A');
+      ReactDOM.render(<B />, otherContainer);
+      expect(otherContainer.textContent).toBe('A');
+    });
+    expect(otherContainer.textContent).toBe('B');
+    expect(calledA).toBe(true);
+    expect(calledB).toBe(true);
   });
 
   it('should not bail out when an update is scheduled from within an event handler', () => {