Make mappedBy lazy.

samples/swarm/DataSource.dart

Index: samples/swarm/DataSource.dart
diff --git a/samples/swarm/DataSource.dart b/samples/swarm/DataSource.dart
index dd7f62d6fcfe236c622764bf573928a2295a7671..658e8a5c1aa2af0f6c26718411238eae7f4a035d 100644
--- a/samples/swarm/DataSource.dart
+++ b/samples/swarm/DataSource.dart
@@ -14,7 +14,7 @@ class Sections extends Collection<Section> {
   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
@@ -87,10 +87,6 @@ class Sections extends Collection<Section> {
   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);
   }