Make mappedBy lazy.

sdk/lib/collection/collections.dart

 // Copyright (c) 2012, 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 [Collections] class implements static methods useful when
  * writing a class that implements [Collection] and the [iterator]
  * method.
  */
 class Collections {
@@ skipping 17 matching lines @@
     return false;
   }
 
   static bool every(Iterable iterable, bool f(o)) {
     for (final e in iterable) {
       if (!f(e)) return false;
     }
     return true;
   }
 
-  static List mappedBy(Iterable source, List destination, f(o)) {
-    for (final e in source) {
-      destination.add(f(e));
-    }
-    return destination;
-  }
-
   static Dynamic reduce(Iterable iterable,
                         Dynamic initialValue,
                         Dynamic combine(Dynamic previousValue, element)) {
     for (final element in iterable) {
       initialValue = combine(initialValue, element);
     }
     return initialValue;
   }
 
   static List where(Iterable source, List destination, bool f(o)) {
@@ skipping 100 matching lines @@
    * Returns true if the specified collection contains the specified object
    * reference.
    */
   static _containsRef(Collection c, Object ref) {
     for (var e in c) {
       if (e === ref) return true;
     }
     return false;
   }
 }