Make mappedBy lazy.

sdk/lib/html/src/_Collections.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.
 
 part of html;
 
 /**
  * The [Collections] class implements static methods useful when
  * writing a class that implements [Collection] and the [iterator]
  * method.
  */
 class _Collections {
   static bool contains(Iterable<Object> iterable, Object element) {
     for (final e in iterable) {
       if (e == element) return true;
     }
     return false;
   }
 
   static void forEach(Iterable<Object> iterable, void f(Object o)) {
     for (final e in iterable) {
       f(e);
     }
   }
 
-  static List mappedBy(Iterable<Object> source,
-                  List<Object> destination,
-                  f(o)) {
-    for (final e in source) {
-      destination.add(f(e));
-    }
-    return destination;
+  static Iterable mappedBy(Iterable<Object> source, f(o)) {
+    return new MappedIterable(source, f);
   }
 
   static bool some(Iterable<Object> iterable, bool f(Object o)) {
     for (final e in iterable) {
       if (f(e)) return true;
     }
     return false;
   }
 
   static bool every(Iterable<Object> iterable, bool f(Object o)) {
     for (final e in iterable) {
       if (!f(e)) return false;
     }
     return true;
   }
 
   static List where(Iterable<Object> source,
                      List<Object> destination,
                      bool f(o)) {
     for (final e in source) {
       if (f(e)) destination.add(e);
     }
     return destination;
   }
 
   static bool isEmpty(Iterable<Object> iterable) {
     return !iterable.iterator.moveNext();
   }
 }