Reorganize barback's source files.

pkg/barback/lib/src/asset/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;
+library barback.asset.asset;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:convert';
 
 import 'asset_id.dart';
 import 'internal_asset.dart';
 
 /// A blob of content.
 ///
@@ skipping 25 matching lines @@
   /// If the asset was created from a [String] the original string is always
   /// returned and [encoding] is ignored. Otherwise, the binary data of the
   /// asset is decoded using [encoding], which defaults to [UTF8].
   Future<String> readAsString({Encoding encoding});
 
   /// Streams the binary contents of the asset.
   ///
   /// If the asset was created from a [String], this returns its UTF-8 encoding.
   Stream<List<int>> read();
 }