Rewrite ReactFiberScheduler for better integration with Scheduler package (#15151)

* Rewrite ReactFiberScheduler

Adds a new implementation of ReactFiberScheduler behind a feature flag.
We will maintain both implementations in parallel until the new one
is proven stable enough to replace the old one.

The main difference between the implementations is that the new one is
integrated with the Scheduler package's priority levels.

* Conditionally add fields to FiberRoot

Some fields only used by the old scheduler, and some by the new.

* Add separate build that enables new scheduler

* Re-enable skipped test

If synchronous updates are scheduled by a passive effect, that work
should be flushed synchronously, even if flushPassiveEffects is
called inside batchedUpdates.

* Passive effects have same priority as render

* Revert ability to cancel the current callback

React doesn't need this anyway because it never schedules callbacks if
it's already rendering.

* Revert change to FiberDebugPerf

Turns out this isn't neccessary.

* Fix ReactFiberScheduler dead code elimination

Should initialize to nothing, then assign the exports conditionally,
instead of initializing to the old exports and then reassigning to the
new ones.

* Don't yield before commit during sync error retry

* Call Scheduler.flushAll unconditionally in tests

Instead of wrapping in enableNewScheduler flag.

Author: acdlite

src/__tests__/ReactTestRenderer-test.js

@@ -9,15 +9,25 @@
 
 'use strict';
 
-const ReactDOM = require('react-dom');
-
-// Isolate test renderer.
-jest.resetModules();
-const React = require('react');
-const ReactCache = require('react-cache');
-const ReactTestRenderer = require('react-test-renderer');
+let ReactDOM;
+let React;
+let ReactCache;
+let ReactTestRenderer;
+let Scheduler;
 
 describe('ReactTestRenderer', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    ReactDOM = require('react-dom');
+
+    // Isolate test renderer.
+    jest.resetModules();
+    React = require('react');
+    ReactCache = require('react-cache');
+    ReactTestRenderer = require('react-test-renderer');
+    Scheduler = require('scheduler');
+  });
+
   it('should warn if used to render a ReactDOM portal', () => {
     const container = document.createElement('div');
     expect(() => {
@@ -62,13 +72,15 @@ describe('ReactTestRenderer', () => {
       const root = ReactTestRenderer.create(<App text="initial" />);
       PendingResources.initial('initial');
       await Promise.resolve();
+      Scheduler.flushAll();
       expect(root.toJSON()).toEqual('initial');
 
       root.update(<App text="dynamic" />);
       expect(root.toJSON()).toEqual('fallback');
 
       PendingResources.dynamic('dynamic');
       await Promise.resolve();
+      Scheduler.flushAll();
       expect(root.toJSON()).toEqual('dynamic');
 
       done();
@@ -88,13 +100,15 @@ describe('ReactTestRenderer', () => {
       const root = ReactTestRenderer.create(<App text="initial" />);
       PendingResources.initial('initial');
       await Promise.resolve();
+      Scheduler.flushAll();
       expect(root.toJSON().children).toEqual(['initial']);
 
       root.update(<App text="dynamic" />);
       expect(root.toJSON().children).toEqual(['fallback']);
 
       PendingResources.dynamic('dynamic');
       await Promise.resolve();
+      Scheduler.flushAll();
       expect(root.toJSON().children).toEqual(['dynamic']);
 
       done();