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`
* @return {object}
   * @private
   */
  _processChildContext: function(currentContext) {
    var Component = this._currentElement.type;
    var inst = this._instance;
    var childContext;
    if (typeof inst.getChildContext === 'function') {
      if (__DEV__) {
        ReactInstrumentation.debugTool.onBeginProcessingChildContext();
        try {
          childContext = inst.getChildContext();
        } finally {
          ReactInstrumentation.debugTool.onEndProcessingChildContext();
        }
      } else {
        childContext = inst.getChildContext();
      }
      invariant(
        typeof Component.childContextTypes === 'object',
        '%s.getChildContext(): childContextTypes must be defined in order to ' +
          'use getChildContext().',
        this.getName() || 'ReactCompositeComponent',
      );
      if (__DEV__) {
        this._checkContextTypes(
          Component.childContextTypes,
          childContext,
          'child context',
        );