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".
if (console.error.calls && console.error.calls.reset) {
    console.error.calls.reset();
  } else {
    spyOn(console, 'error');
  }
  const result = await fn();
  if (
    console.error.calls.count() !== count &&
    console.error.calls.count() !== 0
  ) {
    console.log(
      `We expected ${count} warning(s), but saw ${console.error.calls.count()} warning(s).`,
    );
    if (console.error.calls.count() > 0) {
      console.log(`We saw these warnings:`);
      for (var i = 0; i < console.error.calls.count(); i++) {
        console.log(console.error.calls.argsFor(i)[0]);
      }
    }
  }
  expectDev(console.error.calls.count()).toBe(count);
  return result;
}
function renderIntoDom(reactElement, domElement, forceHydrate, errorCount = 0) {