Core: Hassio component missing after upgrade to 0.90.0b4

[TheZoker]

Today I installed a fresh instance of hassio (v. 2.10) on my pi 3B+. It then pulled HA 0.89.2 automatically.
I immediately updated to 0.90.0b4 without changing any config files.

After the upgrade the hassio: component was completely missing from the configuration.yaml file.
The /hassio/dashboard resulted in a 404 error.

@OttoWinter confirmed this issue:
https://ptb.discordapp.com/channels/330944238910963714/427516175237382144/556896802088157224

The only way to fix this, was to plug the sd card into a linux PC and manually re-add the hassio component.

[OttoWinter]

I'm wondering if this could this have something to do with default_config:.

[cogneato]

Would this default_config change remove an already existing discovery:?
https://github.com/home-assistant/home-assistant/commit/95da6d41f9fe7f2c5d5d30783ba69bf52b75b80c#diff-579ab6a404463a7992f2626b9139dac2

hassio: is not the default on a fresh install. You would have to enter it manually (or restore from snapshot) and then have it removed...

[balloob]

Fixing it in latest beta, loading Hass.io component will no longer rely on discovery being present.