Add a `deepCopy` benchmark (#724)
Add a `deepCopy` benchmark (#724)

`deepCopy` is widely used to convert a frozen (immutable) message into a
mutable message and it's worth tracking the performance of it and
optimizing it.
google/protobuf.dart,dart-lang/dart-protobuf,dart-lang/dart-protobuf,dart-lang/protobuf,dart-lang/protobuf,google/protobuf.dart,dart-lang/protobuf,dart-lang/dart-protobuf,google/protobuf.dart