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**
/**
 * Copyright 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @emails react-core
 */
'use strict';
let ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
let ExecutionEnvironment;
let PropTypes;
let React;
let ReactDOM;
let ReactDOMServer;
let ReactDOMNodeStream;
let ReactTestUtils;
const stream = require('stream');
function asyncReactDOMRender(reactElement, domElement) {
  return new Promise(resolve => {
    ReactDOM.render(reactElement, domElement);
    
    
    resolve();
  });
}
async function expectErrors(fn, count) {