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 
constructor() {
        super();
        click(node);
      }
      render() {
        return null;
      }
    }
    let inst;
    ReactDOM.render([<Example key="a" ref={n => (inst = n)} />], container);
    const node = container.firstChild;
    expect(node.tagName).toEqual('DIV');
    function click(target) {
      ReactTestUtils.Simulate.click(target);
    }
    click(node);
    expect(ops).toEqual(['A']);
    ops = [];
    
    inst.flip();
    click(node);
    expect(ops).toEqual(['B']);
    ops = [];
    
    inst.tick();