Make Assets know their ID.

pkg/barback/lib/src/asset.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;
 
 import 'dart:async';
 import 'dart:io';
 
+import 'asset_id.dart';
+
 /// A blob of content.
 ///
 /// Assets may come from the file system, or as the output of a [Transformer].
 /// They are identified by [AssetId].
 abstract class Asset {
-  factory Asset.fromFile(File file) {
-    return new _FileAsset(file);
+  /// The ID for this asset.
+  final AssetId id;
+
+  Asset(this.id);
+
+  factory Asset.fromFile(AssetId id, File file) {
+    return new _FileAsset(id, file);
   }
 
-  factory Asset.fromString(String content) {
-    return new _StringAsset(content);
+  factory Asset.fromString(AssetId id, String content) {
+    return new _StringAsset(id, content);
   }
 
-  factory Asset.fromPath(String path) {
-    return new _FileAsset(new File(path));
+  factory Asset.fromPath(AssetId id, String path) {
+    return new _FileAsset(id, new File(path));
   }
 
   // TODO(rnystrom): This prevents users from defining their own
   // implementations of Asset. Use serialization package instead.
   factory Asset.deserialize(data) {
     // TODO(rnystrom): Handle errors.
+    var id = new AssetId.parse(data[1]);
     switch (data[0]) {
-      case "file": return new _FileAsset(new File(data[1])); break;
-      case "string": return new _StringAsset(data[1]); break;
+      case "file": return new _FileAsset(id, new File(data[2])); break;
+      case "string": return new _StringAsset(id, data[2]); break;
     }
   }
 
   /// Returns the contents of the asset as a string.
   // TODO(rnystrom): Figure out how binary assets should be handled.
   Future<String> readAsString();
 
   /// Streams the contents of the asset.
   Stream<List<int>> read();
 
   /// Serializes this [Asset] to an object that can be sent across isolates
   /// and passed to [deserialize].
   Object serialize();
 }
 
 /// An asset backed by a file on the local file system.
-class _FileAsset implements Asset {
+class _FileAsset extends Asset {
   final File _file;
-  _FileAsset(this._file);
+  _FileAsset(AssetId id, this._file)
+      : super(id);
 
   Future<String> readAsString() => _file.readAsString();
   Stream<List<int>> read() => _file.openRead();
 
   String toString() => 'File "${_file.path}"';
 
-  Object serialize() => ["file", _file.path];
+  Object serialize() => ["file", id.serialize(), _file.path];
 }
 
 /// An asset whose data is stored in a string.
 // TODO(rnystrom): Have something similar for in-memory binary assets.
-class _StringAsset implements Asset {
+class _StringAsset extends Asset {
   final String _contents;
 
-  _StringAsset(this._contents);
+  _StringAsset(AssetId id, this._contents)
+      : super(id);
 
   Future<String> readAsString() => new Future.value(_contents);
 
   // TODO(rnystrom): Implement this and handle encoding.
   Stream<List<int>> read() => throw new UnimplementedError();
 
   String toString() {
     // Don't show the whole string if it's long.
     var contents = _contents;
     if (contents.length > 40) {
       contents = contents.substring(0, 20) + " ... " +
                  contents.substring(contents.length - 20);
     }
 
     contents = _escape(contents);
     return 'String "$contents"';
   }
 
-  Object serialize() => ["string", _contents];
+  Object serialize() => ["string", id.serialize(), _contents];
 
   String _escape(String string) {
     return string
         .replaceAll("\"", r'\"')
         .replaceAll("\n", r"\n")
         .replaceAll("\r", r"\r")
         .replaceAll("\t", r"\t");
   }
 }