`flutter downgrade` fails if you haven't used `flutter upgrade`:
```
$ flutter downgrade
There is no previously recorded version for channel "stable".
```
It's not clear what actions a user should take from this error message. Here's the new error message:
```
$ flutter downgrade
It looks like you haven't run "flutter upgrade" on channel "stable".
"flutter downgrade" undoes the last "flutter upgrade".
To switch to a specific Flutter version, see: https://flutter.dev/to/switch-flutter-version
```
Depends on https://github.com/flutter/website/pull/11098
3bda455f4e