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`
inst.context = publicContext;
    inst.refs = emptyObject;
    inst.updater = updateQueue;
    this._instance = inst;
    
    ReactInstanceMap.set(inst, this);
    if (__DEV__) {
      
      
      
      warning(
        !inst.getInitialState ||
          inst.getInitialState.isReactClassApproved ||
          inst.state,
        'getInitialState was defined on %s, a plain JavaScript class. ' +
          'This is only supported for classes created using React.createClass. ' +
          'Did you mean to define a state property instead?',
        this.getName() || 'a component',
      );
      warning(
        !inst.getDefaultProps || inst.getDefaultProps.isReactClassApproved,
        'getDefaultProps was defined on %s, a plain JavaScript class. ' +
          'This is only supported for classes created using React.createClass. ' +
          'Use a static property to define defaultProps instead.',
        this.getName() || 'a component',
      );
      warning(
        !inst.propTypes,
        'propTypes was defined as an instance property on %s. Use a static ' +
          'property to define propTypes instead.',