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`
if (shouldUpdate) {
      this._pendingForceUpdate = false;
      
      this._performComponentUpdate(
        nextParentElement,
        nextProps,
        nextState,
        nextContext,
        transaction,
        nextUnmaskedContext,
      );
    } else {
      
      
      this._currentElement = nextParentElement;
      this._context = nextUnmaskedContext;
      inst.props = nextProps;
      inst.state = nextState;
      inst.context = nextContext;
    }
    if (callbacks) {
      for (var j = 0; j < callbacks.length; j++) {
        transaction
          .getReactMountReady()
          .enqueue(callbacks[j], this.getPublicInstance());
      }
    }
  },
  _processPendingState: function(props, context) {
    var inst = this._instance;