Run test script and fix regressions

Fixes the case where there's an uncaught error and the root unmounts.
We implement this by rendering the root as if its child is null. Null is
not usually allowed at the top level, so we need to special case it.

scripts/fiber/tests-failing.txt

@@ -64,10 +64,6 @@ src/renderers/shared/shared/__tests__/ReactEmptyComponent-test.js
 src/renderers/shared/shared/__tests__/ReactMultiChildText-test.js
 * should reorder keyed text nodes
 
-src/renderers/shared/shared/__tests__/ReactStatelessComponent-test.js
-* should warn when stateless component returns array
-* should warn when using non-React functions in JSX
-
 src/renderers/shared/shared/__tests__/ReactUpdates-test.js
 * marks top-level updates
 

scripts/fiber/tests-passing.txt

@@ -544,6 +544,8 @@ src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
 * should bubble events from the portal to the parent
 * should not onMouseLeave when staying in the portal
 * should not crash encountering low-priority tree
+* throws if non-element passed to top-level render
+* throws if something other than false, null, or an element is returned from render
 
 src/renderers/dom/shared/__tests__/CSSProperty-test.js
 * should generate browser prefixes for its `isUnitlessNumber`
@@ -1536,6 +1538,7 @@ src/renderers/shared/shared/__tests__/ReactStatelessComponent-test.js
 * should update stateless component
 * should unmount stateless component
 * should pass context thru stateless component
+* should throw when stateless component returns undefined
 * should throw on string refs in pure functions
 * should warn when given a string ref
 * should warn when given a function ref

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

@@ -1015,8 +1015,10 @@ describe('ReactDOMFiber', () => {
       });
 
       it('throws if non-element passed to top-level render', () => {
+        // FIXME: These assertions pass individually, but they leave React in
+        // an inconsistent state. This suggests an error-handling bug. I'll fix
+        // this in a separate PR.
         const message = 'render(): Invalid component element.';
-
         expect(() => ReactDOM.render(null, container)).toThrow(message, container);
         expect(() => ReactDOM.render(undefined, container)).toThrow(message, container);
         expect(() => ReactDOM.render(false, container)).toThrow(message, container);
@@ -1030,9 +1032,9 @@ describe('ReactDOMFiber', () => {
           return props.children;
         }
 
-        expect(() => ReactDOM.render(<Render>Hi</Render>, container)).toThrow(/Render\.render/);
-        expect(() => ReactDOM.render(<Render>{999}</Render>, container)).toThrow(/Render\.render/);
-        expect(() => ReactDOM.render(<Render>[<div />]</Render>, container)).toThrow(/Render\.render/);
+        expect(() => ReactDOM.render(<Render>Hi</Render>, container)).toThrow(/You may have returned undefined/);
+        expect(() => ReactDOM.render(<Render>{999}</Render>, container)).toThrow(/You may have returned undefined/);
+        expect(() => ReactDOM.render(<Render>[<div />]</Render>, container)).toThrow(/You may have returned undefined/);
       });
     });
   }

src/renderers/shared/fiber/ReactChildFiber.js

@@ -75,6 +75,7 @@ const {
   NoEffect,
   Placement,
   Deletion,
+  Err,
 } = ReactTypeOfSideEffect;
 
 function coerceRef(current: ?Fiber, element: ReactElement) {
@@ -1129,7 +1130,9 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
       if (returnFiber.tag === HostRoot) {
         // Top-level only accepts elements or portals
         invariant(
-          false,
+          // If the root has an error effect, this is an intentional unmount.
+          // Don't throw an error.
+          returnFiber.effectTag & Err,
           'render(): Invalid component element.'
         );
       } else {
@@ -1247,7 +1250,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
           const Component = returnFiber.type;
           invariant(
             false,
-            '%s: Nothing was returned from render. This usually means a ' +
+            '%s(...): Nothing was returned from render. This usually means a ' +
             'return statement is missing. Or, to render nothing, ' +
             'return null.',
             Component.displayName || Component.name || 'Component'

src/renderers/shared/fiber/__tests__/ReactIncremental-test.js

@@ -956,6 +956,7 @@ describe('ReactIncremental', () => {
 
     function Trail() {
       ops.push('Trail');
+      return null;
     }
 
     function App(props) {

src/renderers/shared/fiber/__tests__/ReactIncrementalErrorHandling-test.js

@@ -496,6 +496,7 @@ describe('ReactIncrementalErrorHandling', () => {
       if (props.throw) {
         throw new Error('Hello');
       }
+      return null;
     }
 
     function Foo() {

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

@@ -146,8 +146,13 @@ describe('ReactStatelessComponent', () => {
     expect(function() {
       ReactTestUtils.renderIntoDocument(<div><NotAComponent /></div>);
     }).toThrowError(
-      'NotAComponent(...): A valid React element (or null) must be returned. ' +
-      'You may have returned undefined, an array or some other invalid object.'
+      ReactDOMFeatureFlags.useFiber ?
+        // Fiber gives a more specific error message for undefined because it
+        // supports more return types.
+        'NotAComponent(...): Nothing was returned from render' :
+        // Stack's message is generic.
+        'NotAComponent(...): A valid React element (or null) must be returned. ' +
+        'You may have returned undefined, an array or some other invalid object.'
     );
   });