Reorganize barback's source files.

pkg/barback/lib/src/utils/multiset.dart

 // Copyright (c) 2013, 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 barback.multiset;
+library barback.utils.multiset;
 
 import 'dart:collection';
 
 /// A set of objects where each object can appear multiple times.
 ///
 /// Like a set, this has amortized O(1) insertion, removal, and
 /// existence-checking of elements. Counting the number of copies of an element
 /// in the set is also amortized O(1).
 ///
 /// Distinct elements retain insertion order. Additional copies of an element
@@ skipping 38 matching lines @@
     if (_map[value] == 0) _map.remove(value);
     return true;
   }
 
   /// Returns whether [value] is in the set.
   bool contains(E value) => _map.containsKey(value);
 
   /// Returns the number of copies of [value] in the set.
   int count(E value) => _map.containsKey(value) ? _map[value] : 0;
 }