Rollup manual chunks for vendor bundle

rollup.config.js

import babel from 'rollup-plugin-babel';
import resolve from 'rollup-plugin-node-resolve';
import commonjs from 'rollup-plugin-commonjs';
import replace from 'rollup-plugin-replace';

const env = process.env.NODE_ENV || 'development';

export default {
  input: [
    'src/index.js'
  ],
  output: {
    dir: 'build/js',
    format: 'system',
    name: 'app',
    sourcemap: true,
  },
  manualChunks: {
    'vendor': [
      'node_modules/react/index.js',
      'node_modules/react-dom/index.js'
    ]
  },
  experimentalCodeSplitting: true,
  plugins: [
    babel({
      exclude: 'node_modules/**'
    }),
    resolve(),
    commonjs(),
    replace({
      'process.env.NODE_ENV': JSON.stringify(env)
    })
  ]
};