[Fiber] Make requestIdleCallback() and requestAnimationFrame() shims async

scripts/fiber/tests-passing.txt

@@ -537,6 +537,7 @@ src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
 * findDOMNode should find dom element after expanding a fragment
 * should bubble events from the portal to the parent
 * should not onMouseLeave when staying in the portal
+* should not crash encountering low-priority tree
 
 src/renderers/dom/shared/__tests__/CSSProperty-test.js
 * should generate browser prefixes for its `isUnitlessNumber`

scripts/jest/environment.js

@@ -1,11 +1,13 @@
 /* eslint-disable */
 global.__DEV__ = true;
 
-// For testing DOM Fiber, we synchronously invoke all the scheduling.
+// For testing DOM Fiber.
 global.requestAnimationFrame = function(callback) {
-  callback();
+  setTimeout(callback);
 };
 
 global.requestIdleCallback = function(callback) {
-  callback({ timeRemaining() { return Infinity; } });
+  setTimeout(() => {
+    callback({ timeRemaining() { return Infinity; } });
+  });
 };

src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js

@@ -993,5 +993,14 @@ describe('ReactDOMFiber', () => {
         'leave parent', // Only when we leave the portal does onMouseLeave fire.
       ]);
     });
+
+    it('should not crash encountering low-priority tree', () => {
+      ReactDOM.render(
+        <div hidden={true}>
+          <div />
+        </div>,
+        container
+      );
+    });
   }
 });