Fix all strong-mode warnings.

lib/src/web_socket_handler.dart

Index: lib/src/web_socket_handler.dart
diff --git a/lib/src/web_socket_handler.dart b/lib/src/web_socket_handler.dart
index b8f94a239a59134c760513c10646f92c129817c0..73dcfef30c63106107a553763a25db1b889c6652 100644
--- a/lib/src/web_socket_handler.dart
+++ b/lib/src/web_socket_handler.dart
@@ -5,6 +5,7 @@
 import 'dart:convert';
 
 import 'package:shelf/shelf.dart';
+import 'package:stream_channel/stream_channel.dart';
 import 'package:web_socket_channel/web_socket_channel.dart';
 
 /// A class that exposes a handler for upgrading WebSocket requests.
@@ -64,7 +65,9 @@ class WebSocketHandler {
     }
 
     var protocol = _chooseProtocol(request);
-    request.hijack((channel) {
+    request.hijack((untypedChannel) {
+      var channel = (untypedChannel as StreamChannel).cast/*<List<int>>*/();
+
       var sink = UTF8.encoder.startChunkedConversion(channel.sink);
       sink.add(
           "HTTP/1.1 101 Switching Protocols\r\n"