**Do you want to request a *feature* or report a *bug*?**
bug

**What is the current behavior?**
In normal rendering, you can memoize a class-based component: https://jsfiddle.net/586ea3cx/

With the shallow renderer, it seems like you can't: https://jsfiddle.net/odj217Lv/1/

This might be blocking https://github.com/airbnb/enzyme/pull/1914, in which tests are failing with `Cannot call a class as a function`. (it's tough to repro stuff with the shallow renderer)
if (
      typeof this._instance.UNSAFE_componentWillMount === 'function' ||
      typeof this._instance.componentWillMount === 'function'
    ) {
      const beforeState = this._newState;
      
      
      if (
        typeof element.type.getDerivedStateFromProps !== 'function' &&
        typeof this._instance.getSnapshotBeforeUpdate !== 'function'
      ) {
        if (typeof this._instance.componentWillMount === 'function') {
          this._instance.componentWillMount();
        }
        if (typeof this._instance.UNSAFE_componentWillMount === 'function') {
          this._instance.UNSAFE_componentWillMount();
        }
      }
      
      if (beforeState !== this._newState) {
        this._instance.state = this._newState || emptyObject;
      }
    }
    this._rendered = this._instance.render();
    
    
  }
  _updateClassComponent(element: ReactElement, context: null | Object) {
    const {props, type} = element;
    const oldState = this._instance.state || emptyObject;