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`
},
  /**
   * @protected
   */
  _renderValidatedComponentWithoutOwnerOrContext: function() {
    var inst = this._instance;
    var renderedElement;
    if (__DEV__) {
      renderedElement = measureLifeCyclePerf(
        () => inst.render(),
        this._debugID,
        'render',
      );
    } else {
      renderedElement = inst.render();
    }
    if (__DEV__) {
      
      if (renderedElement === undefined && inst.render._isMockFunction) {
        
        
        renderedElement = null;
      }
    }
    return renderedElement;
  },
  /**
   * @private
   */
  _renderValidatedComponent: function() {