Reorganize barback's source files.

pkg/barback/lib/src/asset_set.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.asset_set;
-
-import 'dart:collection';
-
-import 'asset.dart';
-import 'asset_id.dart';
-
-/// A set of [Asset]s with distinct IDs.
-///
-/// This uses the [AssetId] of each asset to determine uniqueness, so no two
-/// assets with the same ID can be in the set.
-class AssetSet extends IterableBase<Asset> {
-  final _assets = new Map<AssetId, Asset>();
-
-  AssetSet();
-
-  /// Creates a new AssetSet from the contents of [other].
-  ///
-  /// If multiple assets in [other] have the same id, the last one takes
-  /// precedence.
-  AssetSet.from(Iterable<Asset> other) {
-    for (var asset in other) {
-      _assets[asset.id] = asset;
-    }
-  }
-
-  Iterator<Asset> get iterator => _assets.values.iterator;
-
-  int get length => _assets.length;
-
-  /// Gets the [Asset] in the set with [id], or returns `null` if no asset with
-  /// that ID is present.
-  Asset operator[](AssetId id) => _assets[id];
-
-  /// Adds [asset] to the set.
-  ///
-  /// If there is already an asset with that ID in the set, it is replaced by
-  /// the new one. Returns [asset].
-  Asset add(Asset asset) {
-    _assets[asset.id] = asset;
-    return asset;
-  }
-
-  /// Adds [assets] to the set.
-  void addAll(Iterable<Asset> assets) {
-    assets.forEach(add);
-  }
-
-  /// Returns `true` if the set contains [asset].
-  bool contains(Asset asset) {
-    var other = _assets[asset.id];
-    return other == asset;
-  }
-
-  /// Returns `true` if the set contains an [Asset] with [id].
-  bool containsId(AssetId id) {
-    return _assets.containsKey(id);
-  }
-
-  /// If the set contains an [Asset] with [id], removes and returns it.
-  Asset removeId(AssetId id) => _assets.remove(id);
-
-  /// Removes all assets from the set.
-  void clear() {
-    _assets.clear();
-  }
-
-  String toString() => _assets.toString();
-}