Make Assets know their ID.

pkg/barback/test/asset_id_test.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.test.asset_id_test;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 14 matching lines @@
     });
 
     test("throws if the package name is empty '|'", () {
       expect(() => new AssetId.parse("|asset.txt"), throwsFormatException);
     });
 
     test("throws if the path is empty '|'", () {
       expect(() => new AssetId.parse("app|"), throwsFormatException);
     });
   });
+
+  test("equals another ID with the same package and path", () {
+    expect(new AssetId.parse("foo|asset.txt"), equals(
+           new AssetId.parse("foo|asset.txt")));
+
+    expect(new AssetId.parse("foo|asset.txt"), isNot(equals(
+        new AssetId.parse("bar|asset.txt"))));
+
+    expect(new AssetId.parse("foo|asset.txt"), isNot(equals(
+        new AssetId.parse("bar|other.txt"))));
+  });
 }