[analyzer] Generalize invocation of ElementUsageFrontierDetector.

Changes the `BestPracticesVerifier` so that when it invokes
`ElementUsageFrontierDetector` by iterating through a list rather than
invoking a single instance. This prepares for a future CL that will
add another instance of `ElementUsageFrontierDetector` to the list
that detects deprecated uses of `@experimental`.

Change-Id: I6a6a69641173f88163fb9db886daf47175b716aa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/451060
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>

Author: stereotype441