Make mappedBy lazy.

runtime/lib/growable_array.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.
 
 class _GrowableObjectArray<T> implements List<T> {
   factory _GrowableObjectArray._uninstantiable() {
     throw new UnsupportedError(
         "GrowableObjectArray can only be allocated by the VM");
   }
 
@@ skipping 160 matching lines @@
   bool contains(T element) => Collections.contains(this, element);
 
   void forEach(f(T element)) {
     // TODO(srdjan): Use Collections.forEach(this, f);
     // Accessing the list directly improves DeltaBlue performance by 25%.
     for (int i = 0; i < length; i++) {
       f(this[i]);
     }
   }
 
-  Collection mappedBy(f(T element)) {
-    return Collections.mappedBy(this,
-                           new _GrowableObjectArray.withCapacity(length), f);
-  }
+  Iterable mappedBy(f(T element)) => new MappedIterable(this, f);
 
   reduce(initialValue, combine(previousValue, T element)) {
     return Collections.reduce(this, initialValue, combine);
   }
 
   Collection<T> where(bool f(T element)) {
     return Collections.where(this, new _GrowableObjectArray<T>(), f);
   }
 
   bool every(bool f(T element)) {
@@ skipping 23 matching lines @@
   Iterator<T> get iterator => new SequenceIterator<T>(this);
 
   List<T> toList() {
     return new List<T>.from(this);
   }
 
   Set<T> toSet() {
     return new Set<T>.from(this);
   }
 }