Tighten up class usage/checks.
Tighten up class usage/checks.

- Ensure extensions resolution/wiring is happening directly on the
  messageClass (incase someone is doing odd things our out classes).
- Make the extension message check match the other class checks in
  for mergeFrom/isEqual/etc.

google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf