200KB exploration

src-slash-index.js

import ReactDOM from 'react-dom';

webpack.config.js

const path = require('path');
const webpack = require('webpack');

module.exports = {
  entry: path.resolve('src/index.js'),
  output: {
    filename: 'index.js',
    path: path.resolve('dist'),
  },
  plugins: [
    new webpack.optimize.UglifyJsPlugin({}),
    new webpack.DefinePlugin({
      'process.env.NODE_ENV': JSON.stringify('production'),
    })
  ]
}

I wouldn't expect DefinePlugin order to matter, I imagine they operate on different hooks. Tested order locally and it didn't affect size.

Updating w/ JSON.stringify got us down to ~100KB