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

**What is the current behavior?**
When I call setState with a callback in a test using react shallow renderer (via enzyme), the callback gets called and `this.state` is still the old state.

EDIT: This seems limited to componentWillMount

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://jsfiddle.net or similar (template for React 16: https://jsfiddle.net/Luktwrdm/, template for React 15: https://jsfiddle.net/hmbg7e9w/).**

EDIT: Reproduce with this https://github.com/bdwain/setstate-callback-bug

**What is the expected behavior?**
When the setState callback gets called, it should have access to the new state.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
Version 16.
It worked in v15 with enzyme 2.

EDIT: I think this is because in componentWillMount, [this line](https://github.com/facebook/react/blob/master/packages/react-test-renderer/src/ReactShallowRenderer.js#L67) causes the render method in the shallow wrapper to return early, before it updates the state.
let shouldUpdate = true;
    if (this._forcedUpdate) {
      shouldUpdate = true;
      this._forcedUpdate = false;
    } else if (typeof this._instance.shouldComponentUpdate === 'function') {
      shouldUpdate = !!this._instance.shouldComponentUpdate(
        props,
        state,
        context,
      );
    } else if (type.prototype && type.prototype.isPureReactComponent) {
      shouldUpdate =
        !shallowEqual(oldProps, props) || !shallowEqual(oldState, state);
    }
    if (shouldUpdate) {
      if (typeof this._instance.componentWillUpdate === 'function') {
        this._instance.componentWillUpdate(props, state, context);
      }
    }
    this._instance.context = context;
    this._instance.props = props;
    this._instance.state = state;
    if (shouldUpdate) {
      this._rendered = this._instance.render();
    }
    
    
  }
}
class Updater {