Flutter `protoc` does not generate built-in types

Question

/proto/ping.proto:

syntax = "proto3";
import "google/protobuf/empty.proto";
service SvcPing {
  rpc Ping (google.protobuf.Empty) returns (Pong);
}

message Pong {
  string message = 1;
}

Running the command protoc proto/ping.proto --dart_out=grpc:lib/grpc generates the necessary code but without the "google/protobuf/empty.proto":

$ which protoc
/usr/bin/protoc