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 str && str.replace(/\(at .+?:\d+\)/g, '(at **)');
  }
  beforeEach(() => {
    React = require('react');
    ReactDOM = require('react-dom');
    ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
    ReactTestUtils = require('react-dom/test-utils');
  });
  it('should throw on invalid render targets', () => {
    var container = document.createElement('div');
    
    expect(function() {
      ReactDOM.render(<div />, [container]);
    }).toThrowError(/Target container is not a DOM element./);
    expect(function() {
      ReactDOM.render(<div />, null);
    }).toThrowError(/Target container is not a DOM element./);
  });
  it('should throw when supplying a ref outside of render method', () => {
    var instance = <div ref="badDiv" />;
    expect(function() {
      instance = ReactTestUtils.renderIntoDocument(instance);
    }).toThrow();
  });
  it('should warn when children are mutated during render', () => {
    spyOn(console, 'error');
    function Wrapper(props) {