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**
return <div>{props.children}</div>;
    }
    function Bar() {
      return <Indirection><X /></Indirection>;
    }
    function Foo() {
      return <Bar />;
    }
    expect(() => ReactTestUtils.renderIntoDocument(<Foo />)).toThrowError(
      'Element type is invalid: expected a string (for built-in components) ' +
        'or a class/function (for composite components) but got: undefined. ' +
        "You likely forgot to export your component from the file it's " +