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`
} finally {
        ReactCurrentOwner.current = null;
      }
    } else {
      return this._constructComponentWithoutOwner(
        doConstruct,
        publicProps,
        publicContext,
        updateQueue,
      );
    }
  },
  _constructComponentWithoutOwner: function(
    doConstruct,
    publicProps,
    publicContext,
    updateQueue,
  ) {
    var Component = this._currentElement.type;
    if (doConstruct) {
      if (__DEV__) {
        return measureLifeCyclePerf(
          () => new Component(publicProps, publicContext, updateQueue),
          this._debugID,
          'ctor',
        );
      } else {
        return new Component(publicProps, publicContext, updateQueue);
      }
    }