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`
hostParent,
    hostContainerInfo,
    transaction,
    context,
  ) {
    
    if (renderedElement === undefined) {
      renderedElement = this._renderValidatedComponent();
    }
    var nodeType = ReactNodeTypes.getType(renderedElement);
    this._renderedNodeType = nodeType;
    var child = this._instantiateReactComponent(
      renderedElement,
      nodeType !== ReactNodeTypes.EMPTY ,
    );
    this._renderedComponent = child;
    var debugID = 0;
    if (__DEV__) {
      debugID = this._debugID;
    }
    var markup = ReactReconciler.mountComponent(
      child,
      transaction,
      hostParent,
      hostContainerInfo,
      this._processChildContext(context),
      debugID,
    );
    if (__DEV__) {
      if (debugID !== 0) {