[Fiber]Warn when shoulcComponentUpdate returns undefined (facebook#8243)

* [Fiber]Warn when shoulcComponentUpdate returns undefined

* Remove unnecessary fallback

scripts/fiber/tests-failing.txt

@@ -526,7 +526,6 @@ src/renderers/shared/stack/reconciler/__tests__/ReactCompositeComponent-test.js
 * should warn about `setState` on unmounted components
 * should warn about `setState` in render
 * should warn about `setState` in getChildContext
-* should warn when shouldComponentUpdate() returns undefined
 * should pass context to children when not owner
 * should pass context when re-rendered for static child
 * should pass context when re-rendered for static child within a composite component

scripts/fiber/tests-passing.txt

@@ -1016,6 +1016,7 @@ src/renderers/shared/stack/reconciler/__tests__/ReactCompositeComponent-test.js
 * should silently allow `setState`, not call cb on unmounting components
 * should cleanup even if render() fatals
 * should call componentWillUnmount before unmounting
+* should warn when shouldComponentUpdate() returns undefined
 * should warn when componentDidUnmount method is defined
 * should skip update when rerendering element in container
 * only renders once if updated in componentWillReceiveProps

src/renderers/shared/fiber/ReactFiberClassComponent.js

@@ -82,7 +82,18 @@ module.exports = function(scheduleUpdate : (fiber: Fiber) => void) {
 
     const instance = workInProgress.stateNode;
     if (typeof instance.shouldComponentUpdate === 'function') {
-      return instance.shouldComponentUpdate(newProps, newState);
+      const shouldUpdate = instance.shouldComponentUpdate(newProps, newState);
+
+      if (__DEV__) {
+        warning(
+          shouldUpdate !== undefined,
+          '%s.shouldComponentUpdate(): Returned undefined instead of a ' +
+          'boolean value. Make sure to return true or false.',
+          getName(workInProgress, instance)
+        );
+      }
+
+      return shouldUpdate;
     }
 
     const type = workInProgress.type;