Make mappedBy lazy.

sdk/lib/_internal/compiler/implementation/tree/nodes.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.
 
 part of tree;
 
 abstract class Visitor<R> {
   const Visitor();
 
   R visitNode(Node node);
@@ skipping 453 matching lines @@
       if (link.head.getEndToken() != null) return link.head.getEndToken();
       if (link.head.getBeginToken() != null) return link.head.getBeginToken();
     }
     return beginToken;
   }
 
   // ------------------- Iterable methods -------------------------------------
   //
   // TODO(floitsch): these functions should be pulled in through a mixin
   // mechanism.
-  Collection mappedBy(f(Node element)) {
-    List result = [];
-    for (Node element in this) result.add(f(element));
-    return result;
-  }
+  Iterable mappedBy(f(Node element)) => new MappedIterable(this, f);
 
   Collection<Node> where(bool f(Node element)) {
     List result = <Node>[];
     for (Node element in this) if (f(element)) result.add(element);
     return result;
   }
 
   bool contains(Node element) {
     for (Node e in this) {
       if (e == element) return true;
@@ skipping 1557 matching lines @@
  * argument).
  *
  * TODO(ahe): This method is controversial, the team needs to discuss
  * if top-level methods are acceptable and what naming conventions to
  * use.
  */
 initializerDo(Node node, f(Node node)) {
   SendSet send = node.asSendSet();
   if (send != null) return f(send.arguments.head);
 }