Add a failing test for portal child reconciliation

It is failing because portal bails out of update, seeing null in pendingProps.
It is null because we set pendingProps to nextPortal.children, which is null in this test.

scripts/fiber/tests-failing.txt

@@ -18,6 +18,9 @@ src/isomorphic/modern/element/__tests__/ReactJSXElementValidator-test.js
 src/renderers/dom/__tests__/ReactDOMProduction-test.js
 * should throw with an error code in production
 
+src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
+* should reconcile portal children
+
 src/renderers/dom/shared/__tests__/ReactDOM-test.js
 * throws in render() if the mount callback is not a function
 * throws in render() if the update callback is not a function

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

@@ -367,6 +367,82 @@ describe('ReactDOMFiber', () => {
       expect(container.innerHTML).toBe('');
     });
 
+    it('should reconcile portal children', () => {
+      var portalContainer = document.createElement('div');
+
+      ReactDOM.render(
+        <div>
+          {ReactDOM.unstable_createPortal(
+            <div>portal:1</div>,
+            portalContainer
+          )}
+        </div>,
+        container
+      );
+      expect(portalContainer.innerHTML).toBe('<div>portal:1</div>');
+      expect(container.innerHTML).toBe('<div></div>');
+
+      ReactDOM.render(
+        <div>
+          {ReactDOM.unstable_createPortal(
+            <div>portal:2</div>,
+            portalContainer
+          )}
+        </div>,
+        container
+      );
+      expect(portalContainer.innerHTML).toBe('<div>portal:2</div>');
+      expect(container.innerHTML).toBe('<div></div>');
+
+      ReactDOM.render(
+        <div>
+          {ReactDOM.unstable_createPortal(
+            <p>portal:3</p>,
+            portalContainer
+          )}
+        </div>,
+        container
+      );
+      expect(portalContainer.innerHTML).toBe('<p>portal:3</p>');
+      expect(container.innerHTML).toBe('<div></div>');
+
+      ReactDOM.render(
+        <div>
+          {ReactDOM.unstable_createPortal(
+            ['Hi', 'Bye'],
+            portalContainer
+          )}
+        </div>,
+        container
+      );
+      expect(portalContainer.innerHTML).toBe('HiBye');
+      expect(container.innerHTML).toBe('<div></div>');
+
+      ReactDOM.render(
+        <div>
+          {ReactDOM.unstable_createPortal(
+            ['Bye', 'Hi'],
+            portalContainer
+          )}
+        </div>,
+        container
+      );
+      expect(portalContainer.innerHTML).toBe('ByeHi');
+      expect(container.innerHTML).toBe('<div></div>');
+
+      ReactDOM.render(
+        <div>
+          {ReactDOM.unstable_createPortal(
+            null,
+            portalContainer
+          )}
+        </div>,
+        container
+      );
+      expect(portalContainer.innerHTML).toBe('');
+      expect(container.innerHTML).toBe('<div></div>');
+    });
+
     it('should keep track of namespace across portals (simple)', () => {
       assertNamespacesMatch(
         <svg {...expectSVG}>