Restructure YAML package suitable for pub lish

pkg/yaml/lib/constructor.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 yaml;
 
 /// Takes a parsed and composed YAML document (what the spec calls the
 /// "representation graph") and creates native Dart objects that represent that
 /// document.
 class _Constructor extends _Visitor {
@@ skipping 36 matching lines @@
     if (anchors.containsKey(anchored.anchor)) return anchors[anchored.anchor];
   }
 
   /// Records that [value] is the Dart object representing [anchored].
   setAnchor(_Node anchored, value) {
     if (anchored.anchor == null) return value;
     anchors[anchored.anchor] = value;
     return value;
   }
 }