Add IOWebSocketChannel.innerWebSocket

Supersedes dart-archive/web_socket_channel#141 - expose the entire `dart:io` web socket instead of
selectively forwarding more parts of the interface.

This does not cover the case of changing a setting immediately after the
socket is connected. As a followup we may want to add a
`Future<WebSocketChannel> get onConnected` to have a signal on
connection and enable immediately setting options.

Author: natebosch

pkgs/web_socket_channel/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 2.1.0-dev
+
+- Add `IOWebSocketChannel.innerWebSocket` getter to access features not exposed
+  through the shared `WebSocketChannel` interface.
+
 ## 2.0.0
 
 - Support null safety.

pkgs/web_socket_channel/lib/io.dart

@@ -35,7 +35,11 @@ class IOWebSocketChannel extends StreamChannelMixin
   @override
   final WebSocketSink sink;
 
-  // TODO(nweiz): Add a compression parameter after the initial release.
+  /// The underlying [WebSocket], if this channel has connected.
+  ///
+  /// `If the future returned from [WebSocket.connect] has not yet completed, or
+  /// completed as an error, this will be null.
+  WebSocket? get innerWebSocket => _webSocket;
 
   /// Creates a new WebSocket connection.
   ///

pkgs/web_socket_channel/pubspec.yaml

@@ -1,5 +1,5 @@
 name: web_socket_channel
-version: 2.0.0
+version: 2.1.0-dev
 
 description: >-
   StreamChannel wrappers for WebSockets. Provides a cross-platform
@@ -8,7 +8,7 @@ description: >-
 repository: https://github.com/dart-lang/web_socket_channel
 
 environment:
-  sdk: ">=2.12.0-0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 dependencies:
   async: ^2.5.0