This:

```js
createElement(<div />)
```

doesn't currently give a distinguishable message.

https://stackoverflow.com/questions/51110436/wrapping-provider-in-same-component-throws-element-type-is-invalid-expected-a-s

But we can easily detect it by checking for `$$typeof` and in that case suggest something more concrete (e.g. "Did you accidentally export JSX instead of a component?").
static propTypes = {prop: PropTypes.string.isRequired};
      static defaultProps = {prop: null};
      render() {
        return React.createElement('span', null, this.props.prop);
      }
    }
    expect(() =>
      ReactTestUtils.renderIntoDocument(React.createElement(Component)),
    ).toWarnDev(
      'Warning: Failed prop type: The prop `prop` is marked as required in ' +
        '`Component`, but its value is `null`.\n' +
        '    in Component',
    );
  });
  it('should not check the default for explicit null', () => {
    class Component extends React.Component {