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 
ops.push(`Parent:${this.props.step} componentWillUnmount`);
      }
      render() {
        const {step} = this.props;
        return [
          <Child key="a" name={`normal[0]:${step}`} />,
          ReactDOM.createPortal(
            <Child key="b" name={`portal1[0]:${step}`} />,
            portalContainer1,
          ),
          <Child key="c" name={`normal[1]:${step}`} />,
          ReactDOM.createPortal(
            [
              <Child key="d" name={`portal2[0]:${step}`} />,
              <Child key="e" name={`portal2[1]:${step}`} />,
            ],
            portalContainer2,
          ),
        ];
      }
    }
    ReactDOM.render(<Parent step="a" />, container);
    expect(portalContainer1.innerHTML).toBe('<div>portal1[0]:a</div>');
    expect(portalContainer2.innerHTML).toBe(
      '<div>portal2[0]:a</div><div>portal2[1]:a</div>',
    );
    expect(container.innerHTML).toBe(
      '<div>normal[0]:a</div><div>normal[1]:a</div>',