**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.
state = {x: 1};
      render() {
        return <div />;
      }
    }
    var moo = ReactTestUtils.renderIntoDocument(<Moo />);
    
    
    moo.updater.enqueueReplaceState(moo, {y: 2});
    expect('x' in moo.state).toBe(false);
    expect(moo.state.y).toBe(2);
  });
  it('should support objects with prototypes as state', () => {
    var NotActuallyImmutable = function(str) {
      this.str = str;
    };
    NotActuallyImmutable.prototype.amIImmutable = function() {
      return true;
    };
    class Moo extends React.Component {
      state = new NotActuallyImmutable('first');
      
      
      _replaceState = update => this.updater.enqueueReplaceState(this, update);
      render() {
        return <div />;
      }
    }
    var moo = ReactTestUtils.renderIntoDocument(<Moo />);
    expect(moo.state.str).toBe('first');