Better handling of asset collisions in barback.

pkg/barback/lib/src/asset_id.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_id;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as pathos;
 
 /// AssetIDs always use POSIX style paths regardless of the host platform.
 final _posix = new pathos.Builder(style: pathos.Style.posix);
 
 /// Identifies an asset within a package.
-class AssetId {
+class AssetId implements Comparable<AssetId> {
   /// The name of the package containing this asset.
   final String package;
 
   /// The path to the asset relative to the root directory of [package].
   ///
   /// Source (i.e. read from disk) and generated (i.e. the output of a
   /// [Transformer]) assets all have paths. Even intermediate assets that are
   /// generated and then consumed by later transformations will still have
   /// a path used to identify it.
   ///
@@ skipping 39 matching lines @@
         path = data[1];
 
   /// Returns `true` of [other] is an [AssetId] with the same package and path.
   operator ==(other) =>
       other is AssetId &&
       package == other.package &&
       path == other.path;
 
   int get hashCode => package.hashCode ^ path.hashCode;
 
+  int compareTo(AssetId other) {
+    var packageComp = package.compareTo(other.package);
+    if (packageComp != 0) return packageComp;
+    return path.compareTo(other.path);
+  }
+
   /// Returns a new [AssetId] with the same [package] as this one and with the
   /// [path] extended to include [extension].
   AssetId addExtension(String extension) =>
       new AssetId(package, "$path$extension");
 
   /// Returns a new [AssetId] with the same [package] and [path] as this one
   /// but with file extension [newExtension].
   AssetId changeExtension(String newExtension) =>
       new AssetId(package, pathos.withoutExtension(path) + newExtension);
 
   String toString() => "$package|$path";
 
   /// Serializes this [AssetId] to an object that can be sent across isolates
   /// and passed to [deserialize].
   serialize() => [package, path];
 }