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site/try/src/user_option.dart

 // Copyright (c) 2014, 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.
 
 library trydart.userOption;
 
 /// Persistent user-configurable option.
 ///
 /// Options included in [options] in settings.dart will automatically be
 /// included in the settings UI unless [isHidden] is true.
@@ skipping 17 matching lines @@
 
   static var storage;
 
   const UserOption(this.name, {this.isHidden: false});
 
   get value => storage[name];
 
   void set value(newValue) {
     storage[name] = newValue;
   }
+
+  void setIfNotInitialized(newValueEvaluator()) {
+    if (storage[name] == null) {
+      value = newValueEvaluator();
+    }
+  }
 }
 
 class BooleanUserOption extends UserOption {
   const BooleanUserOption(String name, {bool isHidden: false})
       : super(name, isHidden: isHidden);
 
   bool get value => super.value == 'true';
 
   void set value(bool newValue) {
     super.value = '$newValue';
   }
 }
 
 class StringUserOption extends UserOption {
   const StringUserOption(String name, {bool isHidden: false})
       : super(name, isHidden: isHidden);
 
   String get value => super.value == null ? '' : super.value;
 
   void set value(String newValue) {
     super.value = newValue;
   }
 }