React 16 alpha allows returning array in render function but server renderer expects children to always be react elements

```
TypeError: Cannot read property 'toLowerCase' of undefined
    at ReactDOMServerRenderer.renderDOM
```

source
```javascript
function renderDOM(element, context) {
    var tag = element.type.toLowerCase(); // element is array
```

Fiddle: https://jsfiddle.net/84v837e9/160/

**react@16.0.0-alpha.13**
...(ReactDOMFeatureFlags.useFiber
        ? [
            
            'inner 1 render',
            'inner 2 render',
            'ref 1 got null',
            'ref 2 got null',
          ]
        : [
            
            'ref 1 got null',
            'inner 1 render',
            'ref 2 got null',
            'inner 2 render',
          ]),
      'inner 1 componentDidUpdate',
      'ref 1 got instance 1',
      'inner 2 componentDidUpdate',
      'ref 2 got instance 2',
      'outer componentDidUpdate',
      'start unmount',
      'outer componentWillUnmount',
      'ref 1 got null',
      'inner 1 componentWillUnmount',
      'ref 2 got null',
      'inner 2 componentWillUnmount',
    ]);
    
  });
  it('fires the callback after a component is rendered', () => {