
**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.
state = {
        counter: 0,
      };
      render() {
        instance = this;
        return (
          <button ref="button" onClick={this.onClick}>
            {this.state.counter}
          </button>
        );
      }
    }
    const shallowRenderer = createRenderer();
    let result = shallowRenderer.render(<SimpleComponent defaultCount={1} />);
    expect(result.props.children).toEqual(0);
    instance.setState((state, props) => {
      return {counter: props.defaultCount + 1};
    });
    result = shallowRenderer.getRenderOutput();
    expect(result.props.children).toEqual(2);
  });
  it('can setState with a callback', () => {
    let instance;
    class SimpleComponent extends React.Component {