* Remove thread-loader
After the cache is warm, it is possible for thread-loader to hurt speed

* Disable babel output formatting

 - babel-preset-react-app@5.0.0
 - react-app-polyfill@0.1.1
 - react-dev-utils@6.0.1
 - react-error-overlay@5.0.1
 - react-scripts@2.0.1

* Switch back to babel-loader

* Preserve existing caller options. Use Object.assign instead of object spread.

* Updated filename in package.json

* Update comment about cache identifier

* Update macro check to use a regex

* Move macro check regex out of function

* Remove runtime alias hack

* Pass absolute path to preset

* Change comment

* Give a relative path to absolute runtime

* Clean up config

* Tweak again

* Make absolute runtime the default

* Remove runtime package from error overlay

* Add workbox service worker functionality

* Remove debug

* Set workboxConfig for when there isn't a cra config file

* Remove workbox configuration options as c-r-a isn't planning on supporting optional configuration

* Remove c-r-a config path from paths

* Add workbox service worker functionality

* Remove c-r-a config path from paths

* Inline the webpack workbox config

* Use settings reccommended by @jeffposnick

https://github.com/facebook/create-react-app/pull/4169/#issuecomment-422963072

* Fallback to public url index.html, not root

* Add one comment

* Update comment

* Correct regex

 - babel-plugin-named-asset-import@0.2.0
 - babel-preset-react-app@4.0.0
 - confusing-browser-globals@1.0.1
 - create-react-app@2.0.0
 - eslint-config-react-app@3.0.0
 - react-app-polyfill@0.1.0
 - react-dev-utils@6.0.0
 - react-error-overlay@5.0.0
 - react-scripts@2.0.0

* Drop ie 11 support and move polyfills to a new package

* More useful directions for what entry point to use
https://github.com/facebook/create-react-app/pull/5090#discussion_r220313783

* Clear up what file this polyfill goes in
https://github.com/facebook/create-react-app/pull/5090#discussion_r220313980

* Polyfill `window`, not `global`

* Remove proxy polyfill file

* add postcss-preset-env, remove autoprefixer

* limit to stage 4

Adjust the default browsers to be a single list and remove it from the build output; it only affects CSS now

Closes #5020

Closes #4932, Closes #4876

* No lock files prevents the use of yarn upgrade & outdated

* Update deps

* Bump Jest across the board

* Add safe postcss parser

* Upgrade html webpack plugin

* Fix build

This reverts commit b43ad04b.

This reverts commit 2c34d5b6.

Uglify is no longer maintained and has bugs not present in Terser (its successor)

The `formatter` option is incompatible with with `thread-loader`.
The `formatter` option previously accepted a function which was lost during JSON serialization.

* Update jest version

* Update babel-jest version

* Use testURL: http://localhost in jest configs.

* Update to jest version 23.5

This version of jest includes a fix for https://github.com/jsdom/jsdom/issues/2304

* "testURL": "http://localhost" is default with jest v23.5

This fixes the functionality of this dependency for Node 10 and above.

Refs: https://github.com/webpack/webpack-dev-server/pull/1451
Refs: https://github.com/nodejs/node/issues/21665

To analyze Webpack bundles, a "stats" JSON is required.

This PR allows that file to be created and saved to the `build`
directory, so that users can use it with Webpack-specific insight
tools like `webpack-bundle-analyzer` without ejecting their
application.

Updated the README to include details for how to do this.

- [x] Utilize webpack 4 development and production modes
- [x] Upgrade webpack dev server
- [x] Webpack 4 compatible release of thread-loader
- [x] Webpack 4 compatible release of HtmlWebpackPlugin
- [x] Webpack 4 compatible release of SwPrecacheWebpackPlugin
- [x] Webpack 4 compatible release of WebpackManifestPlugin
- [x] Update README
- [x] Update WebpackDevServerUtils
- [x] Update InterpolateHtmlPlugin
- [x] Update ModuleScopePlugin
- [x] Update WatchMissingNodeModulesPlugin
- [x] Move UglifyJS options to webpack 4 optimize
- [x] Move InterpolateHtmlPlugin to make it tapable on HtmlWebpackPlugin
- [x] vendor splitting via splitChunks.splitChunks (https://twitter.com/wSokra/status/969633336732905474)
- [x] long term caching via splitChunks.runtimeChunk (https://twitter.com/wSokra/status/969679223278505985)
- [x] Make sure process.env.NODE_ENV is proxied correctly to `react-error-overlay`
- [x] Implicit webpack.NamedModulesPlugin in dev config as its default in webpack 4
- [x] Disable webpack performance hints as we have our own filesize reporter
- [x] Replace ExtractTextPlugin with MiniCssExtractPlugin
- [x] Switch to css whole file minification via OptimizeCSSAssetsPlugin rather than per module css minification to gain performance

* Update svgr to 1.9.2 in order to reduce install size

* Update sass-loader (#4363)

* Added bootstrap scss test (#4376)

* Updated jest version.