Revert "Fix: flushSync changes priority inside effect (facebook#21122)"

This reverts commit 0e3c7e1.

When called from inside an effect, flushSync cannot synchronously flush
its updates because React is already working. So we fire a warning.

However, we should still change the priority of the updates to sync so
that they flush at the end of the current task.

This only affects useEffect because updates inside useLayoutEffect (and
the rest of the commit phase, like ref callbacks) are already sync.

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

@@ -1142,6 +1142,16 @@ export function unbatchedUpdates<A, R>(fn: (a: A) => R, a: A): R {
 
 export function flushSync<A, R>(fn: A => R, a: A): R {
   const prevExecutionContext = executionContext;
+  if ((prevExecutionContext & (RenderContext | CommitContext)) !== NoContext) {
+    if (__DEV__) {
+      console.error(
+        'flushSync was called from inside a lifecycle method. React cannot ' +
+          'flush when React is already rendering. Consider moving this call to ' +
+          'a scheduler task or micro task.',
+      );
+    }
+    return fn(a);
+  }
   executionContext |= BatchedContext;
 
   const previousPriority = getCurrentUpdatePriority();
@@ -1158,17 +1168,7 @@ export function flushSync<A, R>(fn: A => R, a: A): R {
     // Flush the immediate callbacks that were scheduled during this batch.
     // Note that this will happen even if batchedUpdates is higher up
     // the stack.
-    if ((executionContext & (RenderContext | CommitContext)) === NoContext) {
-      flushSyncCallbacks();
-    } else {
-      if (__DEV__) {
-        console.error(
-          'flushSync was called from inside a lifecycle method. React cannot ' +
-            'flush when React is already rendering. Consider moving this call to ' +
-            'a scheduler task or micro task.',
-        );
-      }
-    }
+    flushSyncCallbacks();
   }
 }
 

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

@@ -1142,6 +1142,16 @@ export function unbatchedUpdates<A, R>(fn: (a: A) => R, a: A): R {
 
 export function flushSync<A, R>(fn: A => R, a: A): R {
   const prevExecutionContext = executionContext;
+  if ((prevExecutionContext & (RenderContext | CommitContext)) !== NoContext) {
+    if (__DEV__) {
+      console.error(
+        'flushSync was called from inside a lifecycle method. React cannot ' +
+          'flush when React is already rendering. Consider moving this call to ' +
+          'a scheduler task or micro task.',
+      );
+    }
+    return fn(a);
+  }
   executionContext |= BatchedContext;
 
   const previousPriority = getCurrentUpdatePriority();
@@ -1158,17 +1168,7 @@ export function flushSync<A, R>(fn: A => R, a: A): R {
     // Flush the immediate callbacks that were scheduled during this batch.
     // Note that this will happen even if batchedUpdates is higher up
     // the stack.
-    if ((executionContext & (RenderContext | CommitContext)) === NoContext) {
-      flushSyncCallbacks();
-    } else {
-      if (__DEV__) {
-        console.error(
-          'flushSync was called from inside a lifecycle method. React cannot ' +
-            'flush when React is already rendering. Consider moving this call to ' +
-            'a scheduler task or micro task.',
-        );
-      }
-    }
+    flushSyncCallbacks();
   }
 }