Fix formatting of invalid element invariant

Message is slightly different for functional and class components.

scripts/fiber/tests-failing.txt

@@ -50,9 +50,6 @@ src/renderers/shared/hooks/__tests__/ReactHostOperationHistoryHook-test.js
 src/renderers/shared/shared/__tests__/ReactComponentLifeCycle-test.js
 * should carry through each of the phases of setup
 
-src/renderers/shared/shared/__tests__/ReactEmptyComponent-test.js
-* should still throw when rendering to undefined
-
 src/renderers/shared/shared/__tests__/ReactMultiChildText-test.js
 * should reorder keyed text nodes
 

scripts/fiber/tests-passing.txt

@@ -1435,6 +1435,7 @@ src/renderers/shared/shared/__tests__/ReactCompositeComponentState-test.js
 
 src/renderers/shared/shared/__tests__/ReactEmptyComponent-test.js
 * should not produce child DOM nodes for null and false
+* should still throw when rendering to undefined
 * should be able to switch between rendering null and a normal tag
 * should be able to switch in a list of children
 * should distinguish between a script placeholder and an actual script tag

src/renderers/shared/fiber/ReactChildFiber.js

@@ -1189,8 +1189,10 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     if (disableNewFiberFeatures) {
       // The new child is not an element. If it's not null or false,
       // and the return fiber is a composite component, throw an error.
+      let componentNameSuffix = '(...)';
       switch (returnFiber.tag) {
         case ClassComponent: {
+          componentNameSuffix = '.render()';
           if (__DEV__) {
             const instance = returnFiber.stateNode;
             if (instance.render._isMockFunction && typeof newChild === 'undefined') {
@@ -1208,10 +1210,11 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
           const Component = returnFiber.type;
           invariant(
             newChild === null || newChild === false,
-            '%s(...): A valid React element (or null) must be ' +
+            '%s%s: 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'
+            Component.displayName || Component.name || 'Component',
+            componentNameSuffix
           );
         }
       }