Make Assets know their ID.

pkg/barback/lib/src/asset.dart

Index: pkg/barback/lib/src/asset.dart
diff --git a/pkg/barback/lib/src/asset.dart b/pkg/barback/lib/src/asset.dart
index 636d468886d3d65ca6bccf6354328e0e51f702b8..83def7408156e78863f86359befafeaf503b80f0 100644
--- a/pkg/barback/lib/src/asset.dart
+++ b/pkg/barback/lib/src/asset.dart
@@ -7,30 +7,38 @@ 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;
     }
   }
 
@@ -47,24 +55,26 @@ abstract class Asset {
 }
 
 /// 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);
 
@@ -83,7 +93,7 @@ class _StringAsset implements Asset {
     return 'String "$contents"';
   }
 
-  Object serialize() => ["string", _contents];
+  Object serialize() => ["string", id.serialize(), _contents];
 
   String _escape(String string) {
     return string