Add an HTML implementation of WebSocketChannel.

test/html_test_server.dart

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:io';
+
+main() async {
+  var server = await HttpServer.bind("localhost", 1234);
+  server.transform(new WebSocketTransformer()).listen((webSocket) {
+    print("connected");
+    webSocket.listen((request) {
+      print("got $request");
+      webSocket.add(request);
+    });
+  });
+}