Add ability to send a ping interval on a WebSocket.

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 370d694f0b04de4d0a50b0a89cb9f5241298d8d0..b67f89e3b3ab20f10b53c5cb638141bc22bf31f3 100644
--- a/sdk/lib/io/websocket.dart
+++ b/sdk/lib/io/websocket.dart
@@ -131,6 +131,22 @@ abstract class WebSocket implements Stream, StreamSink {
   String get closeReason;
 
   /**
+   * Set and get the interval for sending ping signals. If a ping message is not
+   * answered by a pong message from the peer, the `WebSocket` is assumed
+   * disconnected and the connection is closed with a
+   * [WebSocketStatus.GOING_AWAY] close code. When a ping signal is sent, the
+   * pong message must be received within [pingInterval].
+   *
+   * There are never two outstanding pings at any given time, and the next ping
+   * timer starts when the pong is received.
+   *
+   * Set the [pingInterval] to `null` to disable sending ping messages.
+   *
+   * The default value is `null`.
+   */
+  Duration pingInterval;
+
+  /**
    * Closes the web socket connection. Set the optional [code] and [reason]
    * arguments to send close information to the remote peer. If they are
    * omitted, the peer will see [WebSocketStatus.NO_STATUS_RECEIVED] code