With 1.4.0 the golang/protobuf json testing protofiles
are being moved to [an internal package][1]. While this is
technically a breaking change, I doubt the maintainers
are going to reintroduce it, so I've copied the necessary
parts to our own testing protofile.
This allows users of this package and, more importantly,
grpcui to try 1.4.0 without this package having to migrate yet.
When it comes to migrating to 1.4.0, this should also make the
transition easier.
[1]: https://github.com/golang/protobuf/tree/v1.4.0-rc.4/internal/testprotos
153d36db8c