"Environments" section of repo results in "ReferenceError" in new v4 app

[josh-m-sharpe]

Short description of the problem:

I've been attempting to include env variables from .env.dev using this section of the README. However I get the error ReferenceError: process is not defined when viewing my app in a browser.

Steps to reproduce:
I created a repo, so you can see the commits:

https://github.com/josh-m-sharpe/env_vars

Pretty straight forward:

1. new v4 app
2. create src/declarations.d.ts
3. create .env.dev
4. add console.log(process.env.MY_VAR) in src/app/home/home.page.ts
5. ionic serve

Which @ionic/app-scripts version are you using?
4.0