https://codesandbox.io/s/40j50607y0

Rendering a component like:

```js
const Foo = () => <React.Fragment />
```

Throws the error:

> Foo(...): Nothing was returned from render. This usually means a return statement is missing. Or, to render nothing, return null.

The error is confusing because clearly `Foo` _is_ returning a value. I don't know if we want to support rendering empty fragments, but in any case this error is confusing.

If this isn't allowed it should explicitly call out that a component cannot return a fragment with no children.

cc @gaearon 
componentDidMount() {
        ops.push(`${this.props.name} componentDidMount`);
      }
      componentDidUpdate() {
        ops.push(`${this.props.name} componentDidUpdate`);
      }
      componentWillUnmount() {
        ops.push(`${this.props.name} componentWillUnmount`);
      }
      render() {
        return <div>{this.props.name}</div>;
      }
    }
    class Parent extends React.Component {
      componentDidMount() {
        ops.push(`Parent:${this.props.step} componentDidMount`);
      }
      componentDidUpdate() {
        ops.push(`Parent:${this.props.step} componentDidUpdate`);
      }
      componentWillUnmount() {