Use web_socket_channel in the README.

README.md

 A library that implements the [JSON-RPC 2.0 spec][spec].
 
 [spec]: http://www.jsonrpc.org/specification
 
 ## Server
 
 A JSON-RPC 2.0 server exposes a set of methods that can be called by clients.
 These methods can be registered using `Server.registerMethod`:
 
 ```dart
 import "package:json_rpc_2/json_rpc_2.dart" as json_rpc;
 import "package:stream_channel/stream_channel.dart";
+import "package:web_socket_channel/io.dart";
 
 main() async {
-  var socket = await WebSocket.connect('ws://localhost:4321');
-  var server = new json_rpc.Server(new StreamChannel(socket, socket));
+  var socket = IOWebSocketChannel.connect('ws://localhost:4321');
+  var server = new json_rpc.Server(socket);
 
   // Any string may be used as a method name. JSON-RPC 2.0 methods are
   // case-sensitive.
   var i = 0;
   server.registerMethod("count", () {
     // Just return the value to be sent as a response to the client. This can
     // be anything JSON-serializable, or a Future that completes to something
     // JSON-serializable.
     return i++;
   });
@@ skipping 47 matching lines @@
 
 ## Client
 
 A JSON-RPC 2.0 client calls methods on a server and handles the server's
 responses to those method calls. These methods can be called using
 `Client.sendRequest`:
 
 ```dart
 import "package:json_rpc_2/json_rpc_2.dart" as json_rpc;
 import "package:stream_channel/stream_channel.dart";
+import "package:web_socket_channel/html.dart";
 
 main() async {
-  var socket = await WebSocket.connect('ws://localhost:4321');
-  var client = new json_rpc.Client(new StreamChannel(socket, socket));
+  var socket = HtmlWebSocketChannel.connect('ws://localhost:4321');
+  var client = new json_rpc.Client(socket);
 
   // This calls the "count" method on the server. A Future is returned that
   // will complete to the value contained in the server's response.
   client.sendRequest("count").then((result) => print("Count is $result."));
 
   // Parameters are passed as a simple Map or, for positional parameters, an
   // Iterable. Make sure they're JSON-serializable!
   client.sendRequest("echo", {"message": "hello"})
       .then((echo) => print('Echo says "$echo"!'));
 
@@ skipping 16 matching lines @@
 ```
 
 ## Peer
 
 Although JSON-RPC 2.0 only explicitly describes clients and servers, it also
 mentions that two-way communication can be supported by making each endpoint
 both a client and a server. This package supports this directly using the `Peer`
 class, which implements both `Client` and `Server`. It supports the same methods
 as those classes, and automatically makes sure that every message from the other
 endpoint is routed and handled correctly.