Make Assets know their ID.

pkg/barback/lib/src/asset_set.dart

Index: pkg/barback/lib/src/asset_set.dart
diff --git a/pkg/barback/lib/src/asset_set.dart b/pkg/barback/lib/src/asset_set.dart
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+++ b/pkg/barback/lib/src/asset_set.dart
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+// 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:async';
+import 'dart:collection';
+import 'dart:io';
+
+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>();
+
+  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);
+  }
+
+  /// Removes all assets from the set.
+  void clear() {
+    _assets.clear();
+  }
+}