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**
this.setState({text: 'hello, world'});
        }
        render() {
          return <div>{this.state.text}</div>;
        }
      }
      ReactReconcileTransaction.prototype.perform = function() {
        
        throw new Error('Browser reconcile transaction should not be used');
      };
      var markup = ReactDOMServer.renderToString(<Component />);
      expect(markup).toContain('hello, world');
    });
    it('renders with props when using custom constructor', () => {
      class Component extends React.Component {
        constructor() {
          super();
        }
        render() {
          return <div>{this.props.text}</div>;
        }
      }
      var markup = ReactDOMServer.renderToString(
        <Component text="hello, world" />,
      );
      expect(markup).toContain('hello, world');
    });
    it('renders with context when using custom constructor', () => {
      class Component extends React.Component {