Move some core classes to collection library.

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 'dart:collection' show Queue;
 
 import 'deep_equals.dart';
 
 part 'yaml_map.dart';
 part 'model.dart';
 part 'parser.dart';
 part 'visitor.dart';
 part 'composer.dart';
 part 'constructor.dart';
 
@@ skipping 27 matching lines @@
 }
 
 /// An error thrown by the YAML processor.
 class YamlException implements Exception {
   String msg;
 
   YamlException(this.msg);
 
   String toString() => msg;
 }