Make mappedBy lazy.

utils/pub/yaml/yaml.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.
 
 library yaml;
 
 import 'dart:math' as Math;
 
 import 'deep_equals.dart';
 
@@ skipping 26 matching lines @@
  * Loads a stream of documents from a YAML string.
  *
  * The return value is mostly normal Dart objects. However, since YAML mappings
  * support some key types that the default Dart map implementation doesn't
  * (null, NaN, booleans, lists, and maps), all maps in the returned document are
  * YamlMaps. These have a few small behavioral differences from the default Map
  * implementation; for details, see the YamlMap class.
  */
 List loadYamlStream(String yaml) {
   return new _Parser(yaml).l_yamlStream().mappedBy((doc) =>
-      new _Constructor(new _Composer(doc).compose()).construct());
+      new _Constructor(new _Composer(doc).compose()).construct())
+      .toList();
 }
 
 /** An error thrown by the YAML processor. */
 class YamlException implements Exception {
   String msg;
 
   YamlException(this.msg);
 
   String toString() => msg;
 }