Disallow Fiber-only render return types when feature flag is on

Except for portals, which we use in some places.

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

@@ -1002,5 +1002,33 @@ describe('ReactDOMFiber', () => {
         container
       );
     });
+
+    describe('disableNewFiberFeatures', () => {
+      var ReactFeatureFlags = require('ReactFeatureFlags');
+
+      beforeEach(() => {
+        ReactFeatureFlags.disableNewFiberFeatures = true;
+      });
+
+      afterEach(() => {
+        ReactFeatureFlags.disableNewFiberFeatures = false;
+      });
+
+      it('throws if something other than false, null, or an element is returned from render', () => {
+        function Render(props) {
+          return props.children;
+        }
+
+        const message = (
+          'Warning: Render.render(): A valid React element (or null) must ' +
+          'be returned. You may have returned undefined, an array or some ' +
+          'other invalid object.'
+        );
+
+        expect(() => ReactDOM.render(<Render>Hi</Render>, container)).toThrow(message);
+        expect(() => ReactDOM.render(<Render>{999}</Render>, container)).toThrow(message);
+        expect(() => ReactDOM.render(<Render>[<div />]</Render>, container)).toThrow(message);
+      });
+    });
   }
 });

src/renderers/shared/fiber/ReactChildFiber.js

@@ -36,6 +36,7 @@ var ReactTypeOfWork = require('ReactTypeOfWork');
 var emptyObject = require('emptyObject');
 var getIteratorFn = require('getIteratorFn');
 var invariant = require('invariant');
+var ReactFeatureFlags = require('ReactFeatureFlags');
 
 if (__DEV__) {
   var { getCurrentFiberStackAddendum } = require('ReactDebugCurrentFiber');
@@ -1100,6 +1101,39 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     // not as a fragment. Nested arrays on the other hand will be treated as
     // fragment nodes. Recursion happens at the normal flow.
 
+    if (ReactFeatureFlags.disableNewFiberFeatures) {
+      // Support only the subset of return types that Stack supports. Treat
+      // everything else as empty, but log a warning.
+      if (typeof newChild === 'object' && newChild !== null) {
+        switch (newChild.$$typeof) {
+          case REACT_ELEMENT_TYPE:
+            return placeSingleChild(reconcileSingleElement(
+              returnFiber,
+              currentFirstChild,
+              newChild,
+              priority
+            ));
+
+          case REACT_PORTAL_TYPE:
+            return placeSingleChild(reconcileSinglePortal(
+              returnFiber,
+              currentFirstChild,
+              newChild,
+              priority
+            ));
+        }
+      }
+
+      const Component = returnFiber.type;
+      invariant(
+        newChild === null || newChild === false,
+        '%s.render(): A valid React element (or null) must be returned. You ' +
+        'may have returned undefined, an array or some other invalid object.',
+        Component.displayName || Component.name || 'Component'
+      );
+      return deleteRemainingChildren(returnFiber, currentFirstChild);
+    }
+
     if (typeof newChild === 'string' || typeof newChild === 'number') {
       return placeSingleChild(reconcileSingleTextNode(
         returnFiber,