Make the repeating argument to timers optional.

samples/chat/chat_server.dart

Index: samples/chat/chat_server.dart
diff --git a/samples/chat/chat_server.dart b/samples/chat/chat_server.dart
index 5c54027220afb2c7833352c3ac84bd3bf505e27f..6927154c2de25d6103a1c859a93a74f2750d53c4 100644
--- a/samples/chat/chat_server.dart
+++ b/samples/chat/chat_server.dart
@@ -19,6 +19,6 @@ void main() {
 
   // Start a shutdown timer if requested.
   if (stopAfter != null) {
-    new Timer((timer) => serverMain.shutdown(), stopAfter * 1000, false);
+    new Timer((timer) => serverMain.shutdown(), stopAfter * 1000);
   }
 }