Currently, if you define `shouldComponentUpdate` on `React.PureComponent` nothing bad happens and it acts like `React.Component` https://jsfiddle.net/jekmdfva/

We should probably warn in dev since it defaults the point of `PureComponent`
workInProgress: Fiber,
    priorityLevel: PriorityLevel,
  ): boolean {
    const instance = workInProgress.stateNode;
    resetInputPointers(workInProgress, instance);
    let newState = workInProgress.memoizedState;
    let newProps = workInProgress.pendingProps;
    if (!newProps) {
      
      
      newProps = workInProgress.memoizedProps;
      invariant(
        newProps != null,
        'There should always be pending or memoized props. This error is ' +
          'likely caused by a bug in React. Please file an issue.',
      );
    }
    const newUnmaskedContext = getUnmaskedContext(workInProgress);
    const newContext = getMaskedContext(workInProgress, newUnmaskedContext);
    
    
    
    if (
      !checkShouldComponentUpdate(
        workInProgress,
        workInProgress.memoizedProps,
        newProps,
        workInProgress.memoizedState,
        newState,
        newContext,