Add a top-level status library.

lib/status.dart

Index: lib/status.dart
diff --git a/lib/status.dart b/lib/status.dart
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+// 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.
+
+/// Status codes that are defined in the WebSocket spec.
+///
+/// This library is intended to be imported with a prefix.
+///
+/// ```dart
+/// import 'package:web_socket_channel/io.dart';
+/// import 'package:web_socket_channel/status.dart' as status;
+///
+/// main() async {
+///   var channel = await IOWebSocketChannel.connect("ws://localhost:1234");
+///   // ...
+///   channel.close(status.goingAway);
+/// }
+/// ```
+import 'dart:core';
+
+/// The purpose for which the connection was established has been fulfilled.
+const normalClosure = 1000;
+
+/// An endpoint is "going away", such as a server going down or a browser having
+/// navigated away from a page.
+const goingAway = 1001;
+
+/// An endpoint is terminating the connection due to a protocol error.
+const protocolError = 1002;
+
+/// An endpoint is terminating the connection because it has received a type of
+/// data it cannot accept.
+///
+/// For example, an endpoint that understands only text data MAY send this if it
+/// receives a binary message).
+const unsupportedData = 1003;
+
+/// No status code was present.
+///
+/// This **must not** be set explicitly by an endpoint.
+const noStatusReceived = 1005;
+
+/// The connection was closed abnormally.
+///
+/// For example, this is used if the connection was closed without sending or
+/// receiving a Close control frame.
+///
+/// This **must not** be set explicitly by an endpoint.
+const abnormalClosure = 1006;
+
+/// An endpoint is terminating the connection because it has received data
+/// within a message that was not consistent with the type of the message.
+///
+/// For example, the endpoint may have receieved non-UTF-8 data within a text
+/// message.
+const invalidFramePayloadData = 1007;
+
+/// An endpoint is terminating the connection because it has received a message
+/// that violates its policy.
+///
+/// This is a generic status code that can be returned when there is no other
+/// more suitable status code (such as [unsupportedData] or [messageTooBig]), or
+/// if there is a need to hide specific details about the policy.
+const policyViolation = 1008;
+
+/// An endpoint is terminating the connection because it has received a message
+/// that is too big for it to process.
+const messageTooBig = 1009;
+
+/// The client is terminating the connection because it expected the server to
+/// negotiate one or more extensions, but the server didn't return them in the
+/// response message of the WebSocket handshake.
+///
+/// The list of extensions that are needed should appear in the close reason.
+/// Note that this status code is not used by the server, because it can fail
+/// the WebSocket handshake instead.
+const missingMandatoryExtension = 1010;
+
+/// The server is terminating the connection because it encountered an
+/// unexpected condition that prevented it from fulfilling the request.
+const internalServerError = 1011;
+
+/// The connection was closed due to a failure to perform a TLS handshake.
+///
+/// For example, the server certificate may not have been verified.
+///
+/// This **must not** be set explicitly by an endpoint.
+const tlsHandshakeFailed = 1015;