**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.
return <div className="parent">{this.props.children}</div>;
      }
    }
    var div = document.createElement('div');
    var parentInstance = null;
    ReactDOM.render(
      <Parent ref={inst => (parentInstance = inst)}>
        <ChildWithoutContext>
          A1
          <GrandChild>A2</GrandChild>
        </ChildWithoutContext>
        <ChildWithContext>
          B1
          <GrandChild>B2</GrandChild>
        </ChildWithContext>
      </Parent>,
      div,
    );
    parentInstance.setState({
      foo: 'def',
    });
    expect(propChanges).toBe(0);
    expect(contextChanges).toBe(3); 
  });
  it('should disallow nested render calls', () => {
    spyOnDev(console, 'error');
    class Inner extends React.Component {