Sam Rawlins
577a7abf67
This adds validation that plugin diagnostics are configured with one
of the valid values (taken from one of the test cases):
```
plugins:
one:
diagnostics:
code1: ignore
code2: warning
code3: error
code4: info
```
Change-Id: I2a77988c2efe1d9e0fdcd33558ed970fbe4c47c0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/494781
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>