Add deprecated WebSocket constructor.

sdk/lib/io/websocket.dart

Index: sdk/lib/io/websocket.dart
diff --git a/sdk/lib/io/websocket.dart b/sdk/lib/io/websocket.dart
index 386671da5d4ca2053103cf6fd01cfc3579166bd9..327ef3aafff218300dd82a20dcf20ac8326d8305 100644
--- a/sdk/lib/io/websocket.dart
+++ b/sdk/lib/io/websocket.dart
@@ -133,6 +133,10 @@ abstract class WebSocket implements Stream, StreamSink {
                                    {List<String> protocols: const []}) =>
       _WebSocketImpl.connect(url, protocols);
 
+  @Deprecated('This constructor will be removed in Dart 2.0. Use `implements`'
+      ' instead of `extends` if implementing this abstract class.')
+  WebSocket();
+
   /**
    * Creates a WebSocket from an already-upgraded socket.
    *