Add an IO implementation of WebSocketChannel.

lib/src/channel.dart

Index: lib/src/channel.dart
diff --git a/lib/src/channel.dart b/lib/src/channel.dart
index a8219a12e291c5878df2ceadc4e51fd950090973..a4e3ed59fe584f49fcf6118a28b833394aada2ed 100644
--- a/lib/src/channel.dart
+++ b/lib/src/channel.dart
@@ -70,7 +70,7 @@ class WebSocketChannel extends StreamChannelMixin {
   WebSocketSink get sink => new WebSocketSink._(_webSocket);
 
   /// Signs a `Sec-WebSocket-Key` header sent by a WebSocket client as part of
-  /// the [initial handshake].
+  /// the [initial handshake][].
   ///
   /// The return value should be sent back to the client in a
   /// `Sec-WebSocket-Accept` header.