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 
render() {
        return this.props.children;
      }
    }
    class Fragment extends React.Component {
      render() {
        return [
          <Wrapper key="a">
            <div />
          </Wrapper>,
          <span key="b" />,
        ];
      }
    }
    let instance = null;
    ReactDOM.render(<Fragment ref={ref => (instance = ref)} />, container);
    expect(container.childNodes.length).toBe(2);
    const firstNode = ReactDOM.findDOMNode(instance);
    expect(firstNode).toBe(container.firstChild);
    expect(firstNode.tagName).toBe('DIV');
  });
  it('finds the first child even when first child renders null', () => {
    class NullComponent extends React.Component {