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

**What is the current behavior?**
The shallow renderer defaults a class component's state to empty object if it's not set. "Real" renderers don't seem to do this, so a component can render successfully in the shallow renderer, but fail at runtime in the browser with a null reference error.

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem.**
Here is ReactDOM's behavior (crashes reading an uninitialized state value): https://codesandbox.io/s/4jv82zmp0
Here is the shallow renderer behavior (component able to render): https://codesandbox.io/s/pp733nrxzq

**What is the expected behavior?**
They initialize state in the same way.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
React 16. This wasn't an issue in React 15.
constructor(props) {
        super(props);
        this.state = {
          bar: 'bar',
        };
      }
      render() {
        return <div>{`${this.props.foo}:${this.state.bar}`}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    const el = <SimpleComponent foo="foo" />;
    let result = shallowRenderer.render(el);
    expect(result).toEqual(<div>foo:bar</div>);
    const cloned = React.cloneElement(el, {foo: 'baz'});
    result = shallowRenderer.render(cloned);
    expect(result).toEqual(<div>baz:bar</div>);
  });
  it('this.state should be updated on setState callback inside componentWillMount', () => {
    let stateSuccessfullyUpdated = false;
    class Component extends React.Component {