Also, add some scripts to generate the analyzer_testing API, and checkin
the api.txt file for analyzer_testing.
In order to avoid a private analyzer import, I add `isExperimental` and
`hasExperimental` support for `@experimental` annotation, and switch
to using public APIs in `_dumpElement`.
Change-Id: I44319270d4e6083b80973b45933268c472876232
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/426282
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>
Sam Rawlins