Make mappedBy lazy.

samples/swarm/swarm_ui_lib/util/CollectionUtils.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.
 
 typedef num NumericValueSelector<T>(T value);
 
 /**
  * General purpose collection utilities.
  * TODO(jmesserly): make these top level functions?
  */
@@ skipping 20 matching lines @@
       // TODO(jmesserly): we won't need to do this once List
       // implements insertAt
       if (arr is ObservableList) {
         // TODO(jmesserly): shouldn't need to cast after testing instanceof
         ObservableList obs = arr;
         obs.recordListInsert(pos, value);
       }
     }
   }
 
-  // Collection<T> supports most of the ES 5 list methods, but it's missing
-  // map.
-
-  // TODO(jmesserly): we might want a version of this that return an iterable,
-  // however JS, Python and Ruby versions are all eager.
-  static List mappedBy(Iterable source, var mapper) {
-    // TODO(jmesserly): I was trying to set the capacity here, but instead it
-    // seems to create a fixed list. Hence assigning by index below.
-    List result = new List(source is List ? (source as List).length : null);
-    int i = 0;
-    for (final item in source) {
-      result[i++] = mapper(item);
-    }
-    return result;
-  }
-
   /**
    * Finds the item in [source] that matches [test].  Returns null if
    * no item matches.  The typing should be:
    * T find(Iterable<T> source, bool test(T item)), but we don't have generic
    * functions.
    */
   static find(Iterable source, bool test(item)) {
     for (final item in source) {
       if (test(item)) return item;
     }
@@ skipping 63 matching lines @@
   }
 
   // TODO(jmesserly): something like should exist on Map, either a method or a
   // constructor, see bug #5340679
   static void copyMap(Map dest, Map source) {
     for (final k in source.keys) {
       dest[k] = source[k];
     }
   }
 }