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 
const usePortal = function(tree) {
    return ReactDOM.createPortal(tree, document.createElement('div'));
  };
  const assertNamespacesMatch = function(tree) {
    container = document.createElement('div');
    svgEls = [];
    htmlEls = [];
    mathEls = [];
    ReactDOM.render(tree, container);
    svgEls.forEach(el => {
      expect(el.namespaceURI).toBe('http://www.w3.org/2000/svg');
    });
    htmlEls.forEach(el => {
      expect(el.namespaceURI).toBe('http://www.w3.org/1999/xhtml');
    });
    mathEls.forEach(el => {
      expect(el.namespaceURI).toBe('http://www.w3.org/1998/Math/MathML');
    });
    ReactDOM.unmountComponentAtNode(container);
    expect(container.innerHTML).toBe('');
  };
  it('should render one portal', () => {
    const portalContainer = document.createElement('div');
    ReactDOM.render(
      <div>{ReactDOM.createPortal(<div>portal</div>, portalContainer)}</div>,
      container,
    );
    expect(portalContainer.innerHTML).toBe('<div>portal</div>');
    expect(container.innerHTML).toBe('<div></div>');
    ReactDOM.unmountComponentAtNode(container);