Flush sync bug (#16027)

* added flush sync test

* added code to run flushSync with ImmediatePriority

* added code to run flushSync with ImmediatePriority

* fixed flow error

* fixed flow error

Author: lunaruan

packages/react-reconciler/src/SchedulerWithReactIntegration.js

@@ -172,12 +172,15 @@ function flushSyncCallbackQueueImpl() {
     let i = 0;
     try {
       const isSync = true;
-      for (; i < syncQueue.length; i++) {
-        let callback = syncQueue[i];
-        do {
-          callback = callback(isSync);
-        } while (callback !== null);
-      }
+      const queue = syncQueue;
+      runWithPriority(ImmediatePriority, () => {
+        for (; i < queue.length; i++) {
+          let callback = queue[i];
+          do {
+            callback = callback(isSync);
+          } while (callback !== null);
+        }
+      });
       syncQueue = null;
     } catch (error) {
       // If something throws, leave the remaining callbacks on the queue.

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

@@ -55,6 +55,17 @@ describe('ReactSchedulerIntegration', () => {
     }
   }
 
+  it('flush sync has correct priority', () => {
+    function ReadPriority() {
+      Scheduler.unstable_yieldValue(
+        'Priority: ' + getCurrentPriorityAsString(),
+      );
+      return null;
+    }
+    ReactNoop.flushSync(() => ReactNoop.render(<ReadPriority />));
+    expect(Scheduler).toHaveYielded(['Priority: Immediate']);
+  });
+
   it('has correct priority during rendering', () => {
     function ReadPriority() {
       Scheduler.unstable_yieldValue(