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 expectErrors(async () => {
    ExecutionEnvironment.canUseDOM = true;
    await asyncReactDOMRender(reactElement, domElement);
    ExecutionEnvironment.canUseDOM = false;
    return domElement.firstChild;
  }, errorCount);
}
async function renderIntoString(reactElement, errorCount = 0) {
  return await expectErrors(
    () =>
      new Promise(resolve =>
        resolve(ReactDOMServer.renderToString(reactElement)),
      ),
    errorCount,
  );
}
async function serverRender(reactElement, errorCount = 0) {
  const markup = await renderIntoString(reactElement, errorCount);
  var domElement = document.createElement('div');
  domElement.innerHTML = markup;
  return domElement.firstChild;
}
class DrainWritable extends stream.Writable {