Restructure YAML package suitable for pub lish

pkg/yaml/lib/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.
 
 part of yaml;
 
 /// 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.
@@ skipping 90 matching lines @@
       for (var e in obj) {
         hash ^= _hashCode(e, parents);
       }
       return hash;
     }
     return obj.hashCode;
   } finally {
     parents.removeLast();
   }
 }