Observed this on react@16.0.0-beta.5

```js
import React from 'react';
import { renderToString } from 'react-dom/server';

const B = () => 'b';

console.log(renderToString(<div>{'a'}<B />{'c'}</div>));
```

This renders as `<div data-reactroot="">a<!-- -->bc</div>`. If you try to hydrate this you'll get the error: Text content did not match. Server: "bc" Client: "b".
return expectErrors(async () => {
    ExecutionEnvironment.canUseDOM = true;
    await asyncReactDOMRender(reactElement, domElement, forceHydrate);
    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 {