webpack loader: TypeError: Cannot read property 'indexOf' of undefined

### Description

What you are reporting:

Bug (or my own misunderstanding). I'm trying to get a basic create-react-app working with hot-loader via the webpack-loader rather than the babel-loader, since I want to use sucrase rather than babel.

### Expected behavior

What you think should happen:

Adding webpack-loader should work.

### Actual behavior

What actually happens:

Running `yarn start` spits this error:

./node_modules/@hot-loader/react-dom/cjs/react-dom.development.js
TypeError: Cannot read property 'indexOf' of undefined

### Environment

React Hot Loader version:

Run these commands in the project folder and fill in their results:

1. `node -v`: 10.15.1
2. `npm -v`: 6.8.0
3. `yarn -v`: 1.13.0

Then, specify:

1. Operating system: MacOS 10.14.4
2. Browser and version: Chrome 73.0.3683.103

### Reproducible Demo

I just did create-react-app, eject, and added hot-loader:

https://github.com/yang/hot-cra-webpack-loader-repro