Split the large client/server.dart files into smaller pieces. This is in
preparation for splitting the HTTP/2 dependencies into a separate file
and make it easier to implement other transports.
Implements the basic server interoperability tests for gRPC compliance.
Compressed tests, TLS, and authentication are not implemented yet, but
basic test cases pass against the C++ gRPC interoperability client.
What works:
* Client and server can talk to each other, all 4 RPC variants.
* Client can talk to Go gRPC server.
What (probably) doesn't work:
* Anything else.
German Saprykin
Jakob Andersen
Michael Thomsen