Reorganize barback's source files.

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 'dart:convert';
-
-import 'asset_id.dart';
-import 'internal_asset.dart';
-
-/// A blob of content.
-///
-/// Assets may come from the file system, or as the output of a [Transformer].
-/// They are identified by [AssetId].
-///
-/// Custom implementations of [Asset] are not currently supported.
-abstract class Asset {
-  /// The ID for this asset.
-  final AssetId id;
-
-  factory Asset.fromBytes(AssetId id, List<int> bytes) =>
-      new BinaryAsset(id, bytes);
-
-  factory Asset.fromFile(AssetId id, File file) =>
-      new FileAsset(id, file.path);
-
-  factory Asset.fromString(AssetId id, String content) =>
-      new StringAsset(id, content);
-
-  factory Asset.fromPath(AssetId id, String path) =>
-      new FileAsset(id, path);
-
-  factory Asset.fromStream(AssetId id, Stream<List<int>> stream) =>
-      new StreamAsset(id, stream);
-
-  /// Returns the contents of the asset as a string.
-  ///
-  /// 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();
-}