Refactor the barback tests somewhat.

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: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>();
 
+  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.
   ///
@@ skipping 13 matching lines @@
   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();
   }
 }