Get rid of WebSocketChannel.pingInterval.

README.md

 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.
+socket, as well as [`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
@@ skipping 35 matching lines @@
 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) {
     // ...
   });
 }
 ```