Use Portals to test batching across roots (#8508)

This doesn't work by default in Fiber, you have to opt-in with a Portal to
get the explicit ordering guarantees.

scripts/fiber/tests-failing.txt

@@ -110,7 +110,6 @@ src/renderers/shared/shared/__tests__/ReactStatelessComponent-test.js
 * should warn when using non-React functions in JSX
 
 src/renderers/shared/shared/__tests__/ReactUpdates-test.js
-* should queue mount-ready handlers across different roots
 * marks top-level updates
 * throws in setState if the update callback is not a function
 * throws in replaceState if the update callback is not a function

scripts/fiber/tests-passing.txt

@@ -1451,6 +1451,7 @@ src/renderers/shared/shared/__tests__/ReactUpdates-test.js
 * should not reconcile children passed via props
 * should flow updates correctly
 * should share reconcile transaction across different roots
+* should queue mount-ready handlers across different roots
 * should flush updates in the correct order
 * should flush updates in the correct order across roots
 * should queue nested updates

src/renderers/shared/shared/__tests__/ReactUpdates-test.js

@@ -544,6 +544,8 @@ describe('ReactUpdates', () => {
     // componentDidUpdate handlers is called, B's DOM should already have been
     // updated.
 
+    var bContainer = document.createElement('div');
+
     var a;
     var b;
 
@@ -558,7 +560,15 @@ describe('ReactUpdates', () => {
       }
 
       render() {
-        return <div>A{this.state.x}</div>;
+        var portal = null;
+        // If we're using Fiber, we use Portals instead to achieve this.
+        if (ReactDOMFeatureFlags.useFiber) {
+          portal = ReactDOM.unstable_createPortal(
+            <B ref={n => b = n} />,
+            bContainer
+          );
+        }
+        return <div>A{this.state.x}{portal}</div>;
       }
     }
 
@@ -571,7 +581,9 @@ describe('ReactUpdates', () => {
     }
 
     a = ReactTestUtils.renderIntoDocument(<A />);
-    b = ReactTestUtils.renderIntoDocument(<B />);
+    if (!ReactDOMFeatureFlags.useFiber) {
+      ReactDOM.render(<B ref={n => b = n} />, bContainer);
+    }
 
     ReactDOM.unstable_batchedUpdates(function() {
       a.setState({x: 1});