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**
'defined in.\n\nCheck the render method of `Bar`.',
    );
    
    expectDev(console.error.calls.count()).toBe(1);
  });
  it('throws if a plain object is used as a child', () => {
    var children = {
      x: <span />,
      y: <span />,
      z: <span />,
    };
    var element = <div>{[children]}</div>;
    var container = document.createElement('div');
    var ex;
    try {
      ReactDOM.render(element, container);
    } catch (e) {
      ex = e;
    }
    expect(ex).toBeDefined();
    expect(normalizeCodeLocInfo(ex.message)).toBe(
      'Objects are not valid as a React child (found: object with keys ' +
        '{x, y, z}). If you meant to render a collection of children, use ' +
        'an array instead.' +
        
        (ReactDOMFeatureFlags.useFiber ? '\n    in div (at **)' : ''),
    );
  });
  it('throws if a plain object even if it is in an owner', () => {
    class Foo extends React.Component {