Reorganize barback's source files.

pkg/barback/lib/src/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;
-
-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
-/// beyond the first are grouped with the original element.
-///
-/// If multiple equal elements are added, only the first actual object is
-/// retained.
-class Multiset<E> extends IterableBase<E> {
-  /// A map from each element in the set to the number of copies of that element
-  /// in the set.
-  final _map = new Map<E, int>();
-
-  Iterator<E> get iterator {
-    return _map.keys.expand((element) {
-      return new Iterable.generate(_map[element], (_) => element);
-    }).iterator;
-  }
-
-  Multiset()
-      : super();
-
-  /// Creates a multi-set and initializes it using the contents of [other].
-  Multiset.from(Iterable<E> other)
-      : super() {
-    other.forEach(add);
-  }
-
-  /// Adds [value] to the set.
-  void add(E value) {
-    _map.putIfAbsent(value, () => 0);
-    _map[value] += 1;
-  }
-
-  /// Removes one copy of [value] from the set.
-  ///
-  /// Returns whether a copy of [value] was removed, regardless of whether more
-  /// copies remain.
-  bool remove(E value) {
-    if (!_map.containsKey(value)) return false;
-
-    _map[value] -= 1;
-    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;
-}