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`
this._pendingElement = null;
    
    
    this._context = null;
    this._rootNodeID = 0;
    this._topLevelWrapper = null;
    
    
    
    ReactInstanceMap.remove(inst);
    
    
    
    
    
  },
  /**
   * Filters the context object to only contain keys specified in
   * `contextTypes`
   *
   * @param {object} context
   * @return {?object}
   * @private
   */
  _maskContext: function(context) {
    var Component = this._currentElement.type;
    var contextTypes = Component.contextTypes;
    if (!contextTypes) {
      return emptyObject;
    }