Make mappedBy lazy.

samples/swarm/DataSource.dart

 // Copyright (c) 2011, 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.
 
 /** The top-level collection of all sections for a user. */
 // TODO(jimhug): This is known as UserData in the server model.
 class Sections extends Collection<Section> {
   final List<Section> _sections;
 
   Sections(this._sections);
 
   operator [](int i) => _sections[i];
 
   int get length => _sections.length;
 
   List<String> get sectionTitles =>
-    CollectionUtils.mappedBy(_sections, (s) => s.title);
+    _sections.mappedBy((s) => s.title).toList();
 
   void refresh() {
     // TODO(jimhug): http://b/issue?id=5351067
   }
 
   /**
    * Find the Section object that has a given title.
    * This is used to integrate well with [ConveyorView].
    */
   Section findSection(String name) {
@@ skipping 52 matching lines @@
       if (name == _sections[i].title) {
         return i;
       }
     }
     return -1;
   }
 
   List<Section> get sections => _sections;
 
   // Collection<Section> methods:
-  List mappedBy(f(Section element)) {
-    return Collections.mappedBy(this, new List(), f);
-  }
-
   List<Section> where(bool f(Section element)) {
     return Collections.where(this, new List<Section>(), f);
   }
   bool every(bool f(Section element)) => Collections.every(this, f);
   bool some(bool f(Section element)) => Collections.some(this, f);
   void forEach(void f(Section element)) { Collections.forEach(this, f); }
 
   // TODO(jmesserly): this should be a property
   bool get isEmpty => length == 0;
 }
@@ skipping 135 matching lines @@
     final hasThumbnail = decoder.readBool();
     final author = decoder.readString();
     final dateInSeconds = decoder.readInt();
     final snippet = decoder.readString();
     final date =
         new Date.fromMillisecondsSinceEpoch(dateInSeconds*1000, isUtc: true);
     return new Article(source, id, date, title, author, srcUrl, hasThumbnail,
         snippet);
   }
 }