Add ability to send a ping interval on a WebSocket.

tests/standalone/io/web_socket_ping_test.dart

+// Copyright (c) 2013, 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.
+//
+// VMOptions=
+// VMOptions=--short_socket_read
+// VMOptions=--short_socket_write
+// VMOptions=--short_socket_read --short_socket_write
+
+import "package:expect/expect.dart";
+import "dart:async";
+import "dart:io";
+
+
+void testPing(int totalConnections) {
+  HttpServer.bind('localhost', 0).then((server) {
+    server.transform(new WebSocketTransformer()).listen((webSocket) {
+      webSocket.pingInterval = const Duration(milliseconds: 100);
+      webSocket.drain();
+    });
+
+    var futures = [];
+    for (int i = 0; i < totalConnections; i++) {
+      futures.add(
+          WebSocket.connect('ws://localhost:${server.port}').then((webSocket) {
+        webSocket.pingInterval = const Duration(milliseconds: 100);
+        webSocket.drain();
+        new Timer(const Duration(seconds: 1), () {
+          // Should not be closed yet.
+          Expect.equals(null, webSocket.closeCode);
+          webSocket.close();
+        });
+        return webSocket.done;
+      }));
+    }
+    Future.wait(futures).then((_) => server.close());
+  });
+}
+
+
+void main() {
+  testPing(10);
+}