Make Assets know their ID.

pkg/barback/test/asset_test.dart

Index: pkg/barback/test/asset_test.dart
diff --git a/pkg/barback/test/asset_test.dart b/pkg/barback/test/asset_test.dart
new file mode 100644
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+++ b/pkg/barback/test/asset_test.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.test.asset_test;
+
+import 'dart:async';
+import 'dart:io';
+
+import 'package:barback/barback.dart';
+import 'package:unittest/unittest.dart';
+
+import 'utils.dart';
+
+main() {
+  initConfig();
+
+  var id = new AssetId.parse("package|path/to/asset.txt");
+
+  group("Asset.fromFile", () {
+    test("returns an asset with the given ID", () {
+      var asset = new Asset.fromFile(id, new File("asset.txt"));
+      expect(asset.id, equals(id));
+    });
+  });
+
+  group("Asset.fromPath", () {
+    test("returns an asset with the given ID", () {
+      var asset = new Asset.fromPath(id, "asset.txt");
+      expect(asset.id, equals(id));
+    });
+  });
+
+  group("Asset.fromString", () {
+    test("returns an asset with the given ID", () {
+      var asset = new Asset.fromString(id, "content");
+      expect(asset.id, equals(id));
+    });
+  });
+}