Add a Client class to json_rpc_2.

pkg/json_rpc_2/lib/src/server.dart

Index: pkg/json_rpc_2/lib/src/server.dart
diff --git a/pkg/json_rpc_2/lib/src/server.dart b/pkg/json_rpc_2/lib/src/server.dart
index bd243de73230f35d5db3d281b553c74a409995e6..1ba8fccc2f923e337d0773d0a66e14f5b7c0f703 100644
--- a/pkg/json_rpc_2/lib/src/server.dart
+++ b/pkg/json_rpc_2/lib/src/server.dart
@@ -69,6 +69,9 @@ class Server {
       : _streams = new TwoWayStream.withoutJson(
             "Server", requests, "requests", responses, "responses");
 
+  /// Users of the library should not use this constructor.
+  Server.internal(this._streams);
+
   /// Starts listening to the underlying stream.
   ///
   /// Returns a [Future] that will complete when the stream is closed or when it