Reorganize barback's source files.

pkg/barback/test/asset_test.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.test.asset_test;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 import 'dart:isolate';
 
 import 'package:barback/barback.dart';
-import 'package:barback/src/internal_asset.dart';
+import 'package:barback/src/asset/internal_asset.dart';
 import 'package:path/path.dart' as pathos;
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 /// The contents of the test binary file.
 final binaryContents = [0, 1, 2, 3, 4];
 
 main() {
   initConfig();
@@ skipping 235 matching lines @@
     });
   });
 }
 
 void _getAssetBytes(message) {
   var asset = deserializeAsset(message['asset']);
   var builder = asset.read().fold(new BytesBuilder(),
       (builder, chunk) => builder..add(chunk));
   builder.then((builder) => message['replyTo'].send(builder.takeBytes()));
 }