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**
render() {
            return <div>foo</div>;
          }
        }
        checkFooDiv(await render(<ClassComponent />));
      });
      itRenders('factory components', async render => {
        const FactoryComponent = () => {
          return {
            render: function() {
              return <div>foo</div>;
            },
          };
        };
        checkFooDiv(await render(<FactoryComponent />));
      });
    });
    describe('component hierarchies', async function() {
      itRenders('single child hierarchies of components', async render => {
        const Component = props => <div>{props.children}</div>;
        let e = await render(
          <Component>
            <Component>
              <Component>
                <Component />
              </Component>
            </Component>
          </Component>,
        );
        for (var i = 0; i < 3; i++) {