analyzer_plugin: Check in api.txt for current public API

Change-Id: I3a429321608e806244304815ea937fced78665ec
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/439420
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: srawlins