npm install && bundle install

In node_modules/react-native/packager/react-packager/src/polyfills/polyfills.js, you need to modify the Object.assign polyfill because it will complain about Opal prototypes

Changed line 56

    for (var key in nextSource) {
      if (__DEV__) {
        var hasOwnProperty = Object.prototype.hasOwnProperty;
        if (!hasOwnProperty.call(nextSource, key) && key.indexOf('$') == -1) { // changed this line
          throw new TypeError(
            'One of the sources for assign has an enumerable key on the ' +
            'prototype chain. This is an edge case that we do not support. ' +
            'This error is a performance optimization and not spec compliant.'
          );
        }
      }
      target[key] = nextSource[key];
    }

Fire off webpack watch with

bundle exec ./node_modules/.bin/webpack --watch &

Then run with react-native run-ios

• Rather than commenting out the entire clause above, probably need to monkey patch it with an exclusion on all Opal prototypes. Couldn't easily do this in testing though
• React packager is slow to refresh when index.ios.js is a big bundle with opal, etc. in it
• https://github.com/mjohnston/react-native-webpack-server can sort of replace all of this but it's without a maintainer and currently has to fight against the React Native packager