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 any of `_pendingElement`, `_pendingStateQueue`, or `_pendingForceUpdate`
   * is set, update the component.
   *
   * @param {ReactReconcileTransaction} transaction
   * @internal
   */
  performUpdateIfNecessary: function(transaction) {
    if (this._pendingElement != null) {
      ReactReconciler.receiveComponent(
        this,
        this._pendingElement,
        transaction,
        this._context,
      );
    } else if (this._pendingStateQueue !== null || this._pendingForceUpdate) {
      this.updateComponent(
        transaction,
        this._currentElement,
        this._currentElement,
        this._context,
        this._context,
      );
    } else {
      var callbacks = this._pendingCallbacks;
      this._pendingCallbacks = null;
      if (callbacks) {
        for (var j = 0; j < callbacks.length; j++) {
          transaction