Reorganize barback's source files.

pkg/barback/lib/src/asset/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;
+library barback.asset.asset_id;
 
 import 'package:path/path.dart' as pathos;
 
 /// Identifies an asset within a package.
 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].
   ///
@@ skipping 86 matching lines @@
   if (pathos.isAbsolute(path)) {
     throw new ArgumentError('Asset paths must be relative, but got "$path".');
   }
 
   // Normalize path separators so that they are always "/" in the AssetID.
   path = path.replaceAll(r"\", "/");
 
   // Collapse "." and "..".
   return pathos.posix.normalize(path);
 }