This is primarily for access to comments, which would be expected to be available in a protoc plugin.
The implementation has two fiddly aspects:
- We use a Lazy<T> to avoid building the map before cross-linking. An alternative would be to crosslink at the end of the constructor, and remove the calls to CrossLink elsewhere. This would be generally better IMO, but deviate from the Java code.
- The casts to IReadOnlyList<DescriptorBase> are unfortunate. They'll always work, because these lists are always ReadOnlyCollection<T> for a descriptor type... but we can't use IList<DescriptorBase> as that's not covariant, and it's annoyingly fiddly to change the field to be of type ReadOnlyCollection<T>.
unittest_proto3 had been changed in a very backward-incompatible
way which was never going to work with C# as it imports proto2 messages.
This is now a copy of the old file, but with a package name change for
compatibility with the remaining files in src/google/protobuf.
The other moves are for files that are only used by C#.
