Before https://github.com/facebook/react/pull/14917, I used to get this:

<img width="502" alt="screen shot 2019-02-21 at 5 38 03 pm" src="https://user-images.githubusercontent.com/810438/53190100-cbe20d00-3600-11e9-9ffa-b230f3bb78fe.png">

'use strict';
const {exec} = require('child-process-promise');
const {join} = require('path');
const puppeteer = require('puppeteer');
const server = require('pushstate-server');
const theme = require('../theme');
const {logPromise} = require('../utils');
const validate = async () => {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();
  await page.goto('http://localhost:9000/fixtures/packaging');
  try {
    return await page.evaluate(() => {
      const iframes = document.querySelectorAll('iframe');
      if (iframes.length === 0) {
        return 'No iframes were found.';
      }
      for (let i = 0; i < iframes.length; i++) {
        const iframe = iframes[i];
        
        const container = iframe.contentDocument.body.getElementsByTagName(
          'div'
        )[0];
        if (container.textContent !== 'Hello World!') {
          return `Unexpected fixture content, "${container.textContent}"`;
        }
      }
      return null;
    });
  } finally {