Add a shelf_web_socket package.

pkg/shelf_web_socket/lib/shelf_web_socket.dart

Index: pkg/shelf_web_socket/lib/shelf_web_socket.dart
diff --git a/pkg/shelf_web_socket/lib/shelf_web_socket.dart b/pkg/shelf_web_socket/lib/shelf_web_socket.dart
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+++ b/pkg/shelf_web_socket/lib/shelf_web_socket.dart
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+// Copyright (c) 2014, 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.
+
+library shelf_web_socket;
+
+import 'package:shelf/shelf.dart';
+
+import 'src/web_socket_handler.dart';
+
+/// A typedef used to determine if a function takes two arguments or not.
+typedef _BinaryFunction(arg1, arg2);
+
+/// Creates a Shelf handler that upgrades HTTP requests to WebSocket
+/// connections.
+///
+/// Only valid WebSocket upgrade requests are upgraded. If a request doesn't
+/// look like a WebSocket upgrade request, a 404 Not Found is returned; if a
+/// request looks like an upgrade request but is invalid, a 400 Bad Request is
+/// returned; and if a request is a valid upgrade request but has an origin that
+/// doesn't match [allowedOrigins] (see below), a 403 Forbidden is returned.
+/// This means that this can be placed first in a [Cascade] and only upgrade
+/// requests will be handled.
+///
+/// The [onConnection] must take a [CompatibleWebSocket] as its first argument.
+/// It may also take a string, the [WebSocket subprotocol][], as its second
+/// argument. The subprotocol is determined by looking at the client's
+/// `Sec-WebSocket-Protocol` header and selecting the first entry that also
+/// appears in [protocols]. If no subprotocols are shared between the client and
+/// the server, `null` will be passed instead. Note that if [onConnection] takes
+/// two arguments, [protocols] must be passed.
+///
+/// [WebSocket subprotocol]: https://tools.ietf.org/html/rfc6455#section-1.9
+///
+/// If [allowedOrigins] is passed, browser connections will only be accepted if
+/// they're made by a script from one of the given origins. This ensures that
+/// malicious scripts running in the browser are unable to fake a WebSocket
+/// handshake. Note that non-browser programs can still make connections freely.
+/// See also the WebSocket spec's discussion of [origin considerations][].
+///
+/// [origin considerations]: https://tools.ietf.org/html/rfc6455#section-10.2
+Handler webSocketHandler(Function onConnection, {Iterable<String> protocols,
+      Iterable<String> allowedOrigins}) {
+  if (protocols != null) protocols = protocols.toSet();
+  if (allowedOrigins != null) {
+    allowedOrigins = allowedOrigins
+        .map((origin) => origin.toLowerCase()).toSet();
+  }
+
+  if (onConnection is! _BinaryFunction) {
+    if (protocols != null) {
+      throw new ArgumentError("If protocols is non-null, onConnection must "
+          "take two arguments, the WebSocket and the protocol.");
+    }
+
+    var innerOnConnection = onConnection;
+    onConnection = (webSocket, _) => innerOnConnection(webSocket);
+  }
+
+  return new WebSocketHandler(onConnection, protocols, allowedOrigins).handle;
+}