Use fiber by default (#9155)

Affects our builds. But not in tests yet.

scripts/jest/test-framework-setup.js

@@ -7,7 +7,7 @@ jest.mock('ReactNative');
 jest.mock('ReactDOMFeatureFlags', () => {
   const flags = require.requireActual('ReactDOMFeatureFlags');
   return Object.assign({}, flags, {
-    useFiber: flags.useFiber || !!process.env.REACT_DOM_JEST_USE_FIBER,
+    useFiber: false || !!process.env.REACT_DOM_JEST_USE_FIBER,
   });
 });
 jest.mock('ReactFeatureFlags', () => {

src/renderers/dom/shared/ReactDOMFeatureFlags.js

@@ -14,7 +14,7 @@
 var ReactDOMFeatureFlags = {
   fiberAsyncScheduling: false,
   useCreateElement: true,
-  useFiber: false,
+  useFiber: true,
 };
 
 module.exports = ReactDOMFeatureFlags;