Make mappedBy lazy.

sdk/lib/core/sequences.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.
 
 /**
  * An indexed sequence of elements of the same type.
  *
  * This is a primitive interface that any finite integer-indexable
  * sequence can implement.
  * It is intended for data structures where access by index is
@@ skipping 21 matching lines @@
  */
 abstract class SequenceCollection<E> implements Collection<E>, Sequence<E> {
   // The class is intended for use as a mixin as well.
 
   Iterator<E> get iterator => new SequenceIterator(sequence);
 
   void forEach(f(E element)) {
     for (int i = 0; i < this.length; i++) f(this[i]);
   }
 
-  Collection mappedBy(f(E element)) {
-    List result = new List();
-    for (int i = 0; i < this.length; i++) {
-      result.add(f(this[i]));
-    }
-    return result;
-  }
+  Iterable mappedBy(f(E element)) => new MappedIterable(this, f);
 
   bool contains(E value) {
     for (int i = 0; i < sequence.length; i++) {
       if (sequence[i] == value) return true;
     }
     return false;
   }
 
   reduce(initialValue, combine(previousValue, E element)) {
     var value = initialValue;
@@ skipping 152 matching lines @@
   }
   E get current {
     if (0 <= _position && _position < _sequence.length) {
       return _sequence[_position];
     }
     // TODO(floitsch): adapt error-message.
     throw new StateError("No more elements");
   }
 }