Further code review changes for WebSocket.fromUpgradedSocket.

sdk/lib/io/websocket.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 /**
  * Web socket status codes used when closing a web socket connection.
  */
 abstract class WebSocketStatus {
@@ skipping 126 matching lines @@
    * Creates a WebSocket from an already-upgraded socket.
    *
    * The initial WebSocket handshake must have occurred prior to this call. A
    * WebSocket client can automatically perform the handshake using
    * [WebSocket.connect], while a server can do so using
    * [WebSocketTransformer.upgrade]. To manually upgrade an [HttpRequest],
    * [HttpRequest.detachSocket] may be called.
    *
    * [protocol] should be the protocol negotiated by this handshake, if any.
    *
-   * If [serverSide] is `false`, the WebSocket will act as the client and mask
-   * the messages it sends. If it's `true`, it will act as the server and will
-   * not mask its messages.
+   * [serverSide] must be passed explicitly. If it's `false`, the WebSocket will
+   * act as the client and mask the messages it sends. If it's `true`, it will
+   * act as the server and will not mask its messages.
    */
   factory WebSocket.fromUpgradedSocket(Socket socket, {String protocol,
-        bool serverSide: true}) =>
-      new _WebSocketImpl._fromSocket(socket, protocol, serverSide);
+        bool serverSide}) {
+    if (serverSide == null) {
+      throw new ArgumentError("The serverSide argument must be passed "
+          "explicitly to WebSocket.fromUpgradedSocket.");
+    }
+    return new _WebSocketImpl._fromSocket(socket, protocol, serverSide);
+  }
 
   /**
    * Returns the current state of the connection.
    */
   int get readyState;
 
   /**
    * The extensions property is initially the empty string. After the
    * web socket connection is established this string reflects the
    * extensions used by the server.
@@ skipping 41 matching lines @@
    */
   Future addStream(Stream stream);
 }
 
 
 class WebSocketException implements IOException {
   final String message;
   const WebSocketException([this.message = ""]);
   String toString() => "WebSocketException: $message";
 }