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`
var queue = this._pendingStateQueue;
    var replace = this._pendingReplaceState;
    this._pendingReplaceState = false;
    this._pendingStateQueue = null;
    if (!queue) {
      return inst.state;
    }
    if (replace && queue.length === 1) {
      return queue[0];
    }
    var nextState = replace ? queue[0] : inst.state;
    var dontMutate = true;
    for (var i = replace ? 1 : 0; i < queue.length; i++) {
      var partial = queue[i];
      let partialState = typeof partial === 'function'
        ? partial.call(inst, nextState, props, context)
        : partial;
      if (partialState) {
        if (dontMutate) {
          dontMutate = false;
          nextState = Object.assign({}, nextState, partialState);
        } else {
          Object.assign(nextState, partialState);
        }
      }
    }
    return nextState;
  },
  /**
   * Merges new props and state, notifies delegate methods of update and