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**
getChildContext() {
            return {text: this.state.text};
          }
          componentWillMount() {
            this.setState({text: 'foo'});
          }
          render() {
            return <Child />;
          }
        }
        WillMountContext.childContextTypes = {text: PropTypes.string};
        const Child = (props, context) => {
          return <div>{context.text}</div>;
        };
        Child.contextTypes = {text: PropTypes.string};
        const e = await render(<WillMountContext />);
        expect(e.textContent).toBe('foo');
      },
    );
    itThrowsWhenRendering(
      'if getChildContext exists without childContextTypes',
      render => {
        class Component extends React.Component {