Reproduction steps:

1. Go to "C:\Users\user\AppData\Local\Unity" and delete/move everything
2. Go to "C:\ProgramData\Unity" and delete/move everything except ".ulf" files
3. Disconnect from the internet
4. Open "simple-app.zip" project

Expected Result: The project launches
Actual Result: A pop-up appears "Failed to resolve packages: Cannot connect to registry: packages.unity.com."

Reproduced with: 2018.3.0a1, 2018.2.0b8, 2018.1.5f1, 2018.1.0a1
Did not reproduce on: 2017.4.5f1

Note:
2018.2 - "An error occurred while resolving packages: Project has invalid dependencies: com.unity.postporcessing: Cannot connect to registry: packages.unity.com"

Steps 1 and 2 might not be necessary for some