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`
instance.context = getMaskedContext(workInProgress, unmaskedContext);
    if (typeof instance.componentWillMount === 'function') {
      if (__DEV__) {
        startPhaseTimer(workInProgress, 'componentWillMount');
      }
      instance.componentWillMount();
      if (__DEV__) {
        stopPhaseTimer();
      }
      
      
      const updateQueue = workInProgress.updateQueue;
      if (updateQueue !== null) {
        instance.state = beginUpdateQueue(
          workInProgress,
          updateQueue,
          instance,
          state,
          props,
          priorityLevel,
        );
      }
    }
    if (typeof instance.componentDidMount === 'function') {
      workInProgress.effectTag |= Update;
    }
  }
  
  
  function resumeMountClassInstance(