[dartfuzz] Add fuzzer support for extension methods for classes

Change-Id: I3843990921d93727e17dd2593177bdf7af740571
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/123340
Reviewed-by: Aart Bik <ajcbik@google.com>
Commit-Queue: Fizaa Luthra <fizaaluthra@google.com>

Author: Fizaa Luthra