[dart2js] Add closed world proto shaking to Dart2js.

Reachability of proto fields is based on the VM's TFA protobuf_handler:
https://github.com/dart-lang/sdk/blob/main/pkg/vm/lib/transformations/type_flow/protobuf_handler.dart

In summary:
Usages of messages' field accessors are detected and registered. If no accessors for a specific field are used then the field is considered unreachable. In this case the metadata initializer for the field is replaced with a placeholder. Any protobuf messages only reachable from erased metadata initializers are also considered unreachable and are erased.

Note: The protobuf library exposes some reflective accessors which this algorithm does not detect usages of. If a field is only accessed reflectively this algorithm will still erase it and therefore users of these reflective accessors should not enable this protobuf shaking.

Protobuf shaking is disabled by default so this change will have no immediate effect on compilations.

Change-Id: I5ddf749bac4ebdcaf364de1cc9a16395a0ad8050
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/349760
Reviewed-by: Mayank Patke <fishythefish@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>