Fix all strong-mode warnings.

lib/src/asset/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.asset_set;
 
 import 'dart:collection';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 36 matching lines @@
     _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;
-  }
+  bool contains(Object asset) => asset is Asset && _assets[asset.id] == 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();
 }