[analyzer] Annotate experimental and deprecated APIs in api.txt file

It occurred to me that it might make it easier to code review changes
to `api.txt` if it included annotations of when an API was marked
`@experimental` or `@deprecated`.

Change-Id: If52eebe2e5086e92491d53bd9d63d3b18c79901e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/413243
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: stereotype441