Remove didTimeout check from work loop

No longer need this, since we have starvation protection in userspace.

This will also allow us to remove the concept from the Scheduler
package, which is nice because `postTask` doesn't currently support it.

packages/react-reconciler/src/ReactFiberWorkLoop.new.js

@@ -754,7 +754,7 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
 
 // This is the entry point for every concurrent task, i.e. anything that
 // goes through Scheduler.
-function performConcurrentWorkOnRoot(root, didTimeout) {
+function performConcurrentWorkOnRoot(root) {
   // Since we know we're in a React event, we can clear the current
   // event time. The next update will compute a new event time.
   currentEventTime = NoTimestamp;
@@ -794,19 +794,6 @@ function performConcurrentWorkOnRoot(root, didTimeout) {
     return null;
   }
 
-  // TODO: We only check `didTimeout` defensively, to account for a Scheduler
-  // bug where `shouldYield` sometimes returns `true` even if `didTimeout` is
-  // true, which leads to an infinite loop. Once the bug in Scheduler is
-  // fixed, we can remove this, since we track expiration ourselves.
-  if (didTimeout) {
-    // Something expired. Flush synchronously until there's no expired
-    // work left.
-    markRootExpired(root, lanes);
-    // This will schedule a synchronous callback.
-    ensureRootIsScheduled(root, now());
-    return null;
-  }
-
   let exitStatus = renderRootConcurrent(root, lanes);
 
   if (

packages/react-reconciler/src/ReactFiberWorkLoop.old.js

@@ -741,7 +741,7 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
 
 // This is the entry point for every concurrent task, i.e. anything that
 // goes through Scheduler.
-function performConcurrentWorkOnRoot(root, didTimeout) {
+function performConcurrentWorkOnRoot(root) {
   // Since we know we're in a React event, we can clear the current
   // event time. The next update will compute a new event time.
   currentEventTime = NoTimestamp;
@@ -781,19 +781,6 @@ function performConcurrentWorkOnRoot(root, didTimeout) {
     return null;
   }
 
-  // TODO: We only check `didTimeout` defensively, to account for a Scheduler
-  // bug where `shouldYield` sometimes returns `true` even if `didTimeout` is
-  // true, which leads to an infinite loop. Once the bug in Scheduler is
-  // fixed, we can remove this, since we track expiration ourselves.
-  if (didTimeout) {
-    // Something expired. Flush synchronously until there's no expired
-    // work left.
-    markRootExpired(root, lanes);
-    // This will schedule a synchronous callback.
-    ensureRootIsScheduled(root, now());
-    return null;
-  }
-
   let exitStatus = renderRootConcurrent(root, lanes);
 
   if (

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

@@ -446,8 +446,6 @@ describe(
       React = require('react');
       ReactNoop = require('react-noop-renderer');
       Scheduler = require('scheduler');
-
-      React = require('react');
     });
 
     afterEach(() => {
@@ -531,6 +529,14 @@ describe(
 
         // Expire the task
         Scheduler.unstable_advanceTime(10000);
+        // Scheduling a new update is a trick to force the expiration to kick
+        // in. We don't check if a update has been starved at the beginning of
+        // working on it, since there's no point — we're already working on it.
+        // We only check before yielding to the main thread (to avoid starvation
+        // by other main thread work) or when receiving an update (to avoid
+        // starvation by incoming updates).
+        ReactNoop.render(<App />);
+
         // Because the render expired, React should finish the tree without
         // consulting `shouldYield` again
         expect(Scheduler).toFlushExpired(['B', 'C']);