Add an IO implementation of WebSocketChannel.

README.md

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+The `web_socket_channel` package provides [`StreamChannel`][stream_channel]
+wrappers for WebSocket connections. It provides a cross-platform
+[`WebSocketChannel`][WebSocketChannel] API, a cross-platform implementation of
+that API that communicates over an underlying [`StreamChannel`][stream_channel],
+and [an implementation][IOWebSocketChannel] that wraps `dart:io`'s `WebSocket`
+class.
+
+[stream_channel]: https://pub.dartlang.org/packages/stream_channel
+[WebSocketChannel]: https://www.dartdocs.org/documentation/web_socket_channel/latest/web_socket_channel/WebSocketChannel-class.html
+[IOWebSocketChannel]: https://www.dartdocs.org/documentation/web_socket_channel/latest/io/IOWebSocketChannel-class.html
+
+## `WebSocketChannel`
+
+The [`WebSocketChannel`][WebSocketChannel] class's most important role is as the
+interface for WebSocket stream channels across all implementations and all
+platforms. In addition to the base `StreamChannel` interface, it adds a
+[`protocol`][protocol] getter that returns the negotiated protocol for the
+socket; a [`pingInterval`][pingInterval] property that allows you to control the
+socket's keep-alive behavior; and [`closeCode`][closeCode] and
+[`closeReason`][closeReason] getters that provide information about why the
+socket closed.
+
+[protocol]: https://www.dartdocs.org/documentation/web_socket_channel/latest/web_socket_channel/WebSocketChannel/protocol.html
+[pingInterval]: https://www.dartdocs.org/documentation/web_socket_channel/latest/web_socket_channel/WebSocketChannel/pingInterval.html
+[closeCode]: https://www.dartdocs.org/documentation/web_socket_channel/latest/web_socket_channel/WebSocketChannel/closeCode.html
+[closeReason]: https://www.dartdocs.org/documentation/web_socket_channel/latest/web_socket_channel/WebSocketChannel/closeReason.html
+
+The channel's [`sink` property][sink] is also special. It returns a
+[`WebSocketSink`][WebSocketSink], which is just like a `StreamSink` except that
+its [`close()`][sink.close] method supports optional `closeCode` and
+`closeReason` parameters. These parameters allow the caller to signal to the
+other socket exactly why they're closing the connection.
+
+[sink]: https://www.dartdocs.org/documentation/web_socket_channel/latest/web_socket_channel/WebSocketChannel/sink.html
+[WebSocketSink]: https://www.dartdocs.org/documentation/web_socket_channel/latest/web_socket_channel/WebSocketSink-class.html
+[sink.close]: https://www.dartdocs.org/documentation/web_socket_channel/latest/web_socket_channel/WebSocketSink/close.html
+
+`WebSocketChannel` also works as a cross-platform implementation of the
+WebSocket protocol. Because it can't initiate or handle HTTP requests in a
+cross-platform way, the [`new WebSocketChannel()` constructor][new] takes an
+underlying [`StreamChannel`][stream_channel] over which it communicates using
+the WebSocket protocol. It also provides the static [`signKey()`][signKey]
+method to make it easier to implement the [initial WebSocket handshake][]. These
+are used in the [`shelf_web_socket`][shelf_web_socket] package to support
+WebSockets in a cross-platform way.
+
+[new]: https://www.dartdocs.org/documentation/web_socket_channel/latest/web_socket_channel/WebSocketChannel/WebSocketChannel.html
+[signKey]: https://www.dartdocs.org/documentation/web_socket_channel/latest/web_socket_channel/WebSocketChannel/signKey.html
+[initial WebSocket handshake]: https://tools.ietf.org/html/rfc6455#section-4.2.2
+[shelf_web_socket]: https://pub.dartlang.org/packages/shelf_web_socket
+
+## `IOWebSocketChannel`
+
+The [`IOWebSocketChannel`][IOWebSocketChannel] class wraps
+[`dart:io`'s `WebSocket` class][io.WebSocket]. Because it imports `dart:io`, it
+has its own library, `package:web_socket_channel/io.dart`. This allows the main
+`WebSocketChannel` class to be available on all platforms.
+
+[io.WebSocket]: https://api.dartlang.org/latest/dart-io/WebSocket-class.html
+
+An `IOWebSocketChannel` can be created by passing a `dart:io` WebSocket to
+[its constructor][new IOWebSocketChannel]. It's more common to want to connect
+directly to a `ws://` or `wss://` URL, in which case
+[`new IOWebSocketChannel.connect()`][IOWebSocketChannel.connect] should be used.
+
+[new IOWebSocketChannel]: https://www.dartdocs.org/documentation/web_socket_channel/latest/io/IOWebSocketChannel/IOWebSocketChannel.html
+[IOWebSocketChannel.connect]: https://www.dartdocs.org/documentation/web_socket_channel/latest/io/IOWebSocketChannel/IOWebSocketChannel.connect.html
+
+```dart
+import 'package:web_socket_channel/io.dart';
+
+main() async {
+  var channel = new IOWebSocketChannel.connect("ws://localhost:8181");
+  channel.sink.add("connected!");
+  channel.sink.listen((message) {
+    // ...
+  });
+}
+```