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 <div onClick="woops" />;
      }
    }
    expect(() => ReactDOM.render(<Example />, container)).toWarnDev(
      'Expected `onClick` listener to be a function, instead got a value of `string` type.\n' +
        '    in div (at **)\n' +
        '    in Example (at **)',
    );
  });
  it('should warn with a special message for `false` event listeners', () => {
    class Example extends React.Component {