Make mappedBy lazy.

utils/pub/yaml/model.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.
 
 // This file contains the node classes for the internal representations of YAML
 // documents. These nodes are used for both the serialization tree and the
 // representation graph.
 
 /** A tag that indicates the type of a YAML node. */
 class _Tag {
@@ skipping 70 matching lines @@
   bool operator ==(other) {
     // Should be super != other; bug 2554
     if (!(super == other) || other is! _SequenceNode) return false;
     if (content.length != other.content.length) return false;
     for (var i = 0; i < content.length; i++) {
       if (content[i] != other.content[i]) return false;
     }
     return true;
   }
 
-  String toString() => '$tag [${Strings.join(content.mappedBy((e) => '$e'), ', ')}]';
+  String toString() =>
+      '$tag [${Strings.join(content.mappedBy((e) => '$e'), ', ')}]';
 
   int get hashCode => super.hashCode ^ _hashCode(content);
 
   visit(_Visitor v) => v.visitSequence(this);
 }
 
 /** An alias node is a reference to an anchor. */
 class _AliasNode extends _Node {
   _AliasNode(String anchor) : super(new _Tag.scalar(_Tag.yaml("str")), anchor);
 
@@ skipping 127 matching lines @@
   String toString() {
     var strContent = Strings.join(content.keys.
         mappedBy((k) => '${k}: ${content[k]}'), ', ');
     return '$tag {$strContent}';
   }
 
   int get hashCode => super.hashCode ^ _hashCode(content);
 
   visit(_Visitor v) => v.visitMapping(this);
 }