Terminate comment with period.

Author: Kasper Lund

demo/todo/web/main.dart

@@ -5,7 +5,7 @@ import 'todo.dart';
 
 import 'dart:html';
 
-// Everything in the 'todo' library should be preserved by MirrorsUsed
+// Everything in the 'todo' library should be preserved by MirrorsUsed.
 @MirrorsUsed(
     targets: const ['todo'],
     override: '*')