Make mappedBy lazy.

utils/pub/yaml/yaml_map.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 class wraps behaves almost identically to the normal Dart Map
  * implementation, with the following differences:
  *
  * * It allows null, NaN, boolean, list, and map keys.
  * * It defines `==` structurally. That is, `yamlMap1 == yamlMap2` if they have
@@ skipping 11 matching lines @@
 
   bool containsValue(value) => _map.containsValue(value);
   bool containsKey(key) => _map.containsKey(_wrapKey(key));
   operator [](key) => _map[_wrapKey(key)];
   operator []=(key, value) { _map[_wrapKey(key)] = value; }
   putIfAbsent(key, ifAbsent()) => _map.putIfAbsent(_wrapKey(key), ifAbsent);
   remove(key) => _map.remove(_wrapKey(key));
   void clear() => _map.clear();
   void forEach(void f(key, value)) =>
     _map.forEach((k, v) => f(_unwrapKey(k), v));
-  Collection get keys => _map.keys.mappedBy(_unwrapKey);
+  Collection get keys => _map.keys.mappedBy(_unwrapKey).toList();
   Collection get values => _map.values;
   int get length => _map.length;
   bool get isEmpty => _map.isEmpty;
   String toString() => _map.toString();
 
   int get hashCode => _hashCode(_map);
 
   bool operator ==(other) {
     if (other is! YamlMap) return false;
     return deepEquals(this, other);
@@ skipping 62 matching lines @@
       for (var e in obj) {
         hash ^= _hashCode(e, parents);
       }
       return hash;
     }
     return obj.hashCode;
   } finally {
     parents.removeLast();
   }
 }