[Mock Scheduler] Mimic browser's advanceTime (#17967)

The mock Scheduler that we use in our tests has its own fake timer
implementation. The `unstable_advanceTime` method advances the timeline.

Currently, a call to `unstable_advanceTime` will also flush any pending
expired work. But that's not how it works in the browser: when a timer
fires, the corresponding task is added to the Scheduler queue. However,
we will still wait until the next message event before flushing it.

This commit changes `unstable_advanceTime` to more closely resemble the
browser behavior, by removing the automatic flushing of expired work.

```js
// Before this commit
Scheduler.unstable_advanceTime(ms);

// Equivalent behavior after this commit
Scheduler.unstable_advanceTime(ms);
Scheduler.unstable_flushExpired();
```

The general principle is to prefer separate APIs for scheduling tasks
and flushing them.

This change does not affect any public APIs. `unstable_advanceTime` is
only used by our own test suite. It is not used by `act`.

However, we may need to update tests in www, like Relay's.

packages/react-dom/src/events/__tests__/ChangeEventPlugin-test.internal.js

@@ -779,6 +779,7 @@ describe('ChangeEventPlugin', () => {
 
           // 3s should be enough to expire the updates
           Scheduler.unstable_advanceTime(3000);
+          expect(Scheduler).toFlushExpired([]);
           expect(container.textContent).toEqual('hovered');
         });
       },

packages/react-reconciler/src/__tests__/ReactIncrementalUpdates-test.internal.js

@@ -540,7 +540,7 @@ describe('ReactIncrementalUpdates', () => {
 
       // All the updates should render and commit in a single batch.
       Scheduler.unstable_advanceTime(10000);
-      expect(Scheduler).toHaveYielded(['Render: goodbye']);
+      expect(Scheduler).toFlushExpired(['Render: goodbye']);
       // Passive effect
       expect(Scheduler).toFlushAndYield(['Commit: goodbye']);
     });
@@ -645,7 +645,7 @@ describe('ReactIncrementalUpdates', () => {
 
       // All the updates should render and commit in a single batch.
       Scheduler.unstable_advanceTime(10000);
-      expect(Scheduler).toHaveYielded([
+      expect(Scheduler).toFlushExpired([
         'Render: goodbye',
         'Commit: goodbye',
         'Render: goodbye',

packages/react-reconciler/src/__tests__/ReactSuspenseWithNoopRenderer-test.internal.js

@@ -848,7 +848,7 @@ describe('ReactSuspenseWithNoopRenderer', () => {
       </Suspense>,
     );
     Scheduler.unstable_advanceTime(10000);
-    expect(Scheduler).toHaveYielded([
+    expect(Scheduler).toFlushExpired([
       'Suspend! [A]',
       'Suspend! [B]',
       'Loading...',
@@ -987,7 +987,7 @@ describe('ReactSuspenseWithNoopRenderer', () => {
     Scheduler.unstable_advanceTime(10000);
     jest.advanceTimersByTime(10000);
 
-    expect(Scheduler).toHaveYielded([
+    expect(Scheduler).toFlushExpired([
       'Suspend! [goodbye]',
       'Loading...',
       'Commit: goodbye',

packages/scheduler/src/__tests__/Scheduler-test.js

@@ -117,14 +117,14 @@ describe('Scheduler', () => {
 
     // Advance by just a bit more to expire the user blocking callbacks
     Scheduler.unstable_advanceTime(1);
-    expect(Scheduler).toHaveYielded([
+    expect(Scheduler).toFlushExpired([
       'B (did timeout: true)',
       'C (did timeout: true)',
     ]);
 
     // Expire A
     Scheduler.unstable_advanceTime(4600);
-    expect(Scheduler).toHaveYielded(['A (did timeout: true)']);
+    expect(Scheduler).toFlushExpired(['A (did timeout: true)']);
 
     // Flush the rest without expiring
     expect(Scheduler).toFlushAndYield([
@@ -140,7 +140,7 @@ describe('Scheduler', () => {
     expect(Scheduler).toHaveYielded([]);
 
     Scheduler.unstable_advanceTime(1);
-    expect(Scheduler).toHaveYielded(['A']);
+    expect(Scheduler).toFlushExpired(['A']);
   });
 
   it('continues working on same task after yielding', () => {
@@ -217,7 +217,7 @@ describe('Scheduler', () => {
 
     // Advance time by just a bit more. This should expire all the remaining work.
     Scheduler.unstable_advanceTime(1);
-    expect(Scheduler).toHaveYielded(['C', 'D']);
+    expect(Scheduler).toFlushExpired(['C', 'D']);
   });
 
   it('continuations are interrupted by higher priority work', () => {
@@ -705,7 +705,7 @@ describe('Scheduler', () => {
 
       // Now it expires
       Scheduler.unstable_advanceTime(1);
-      expect(Scheduler).toHaveYielded(['A']);
+      expect(Scheduler).toFlushExpired(['A']);
     });
 
     it('cancels a delayed task', () => {

packages/scheduler/src/forks/SchedulerHostConfig.mock.js

@@ -201,13 +201,10 @@ export function unstable_yieldValue(value: mixed): void {
 
 export function unstable_advanceTime(ms: number) {
   currentTime += ms;
-  if (!isFlushing) {
-    if (scheduledTimeout !== null && timeoutTime <= currentTime) {
-      scheduledTimeout(currentTime);
-      timeoutTime = -1;
-      scheduledTimeout = null;
-    }
-    unstable_flushExpired();
+  if (scheduledTimeout !== null && timeoutTime <= currentTime) {
+    scheduledTimeout(currentTime);
+    timeoutTime = -1;
+    scheduledTimeout = null;
   }
 }