Reorganize barback's source files.

pkg/barback/lib/src/serialize.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.serialize;
 
 import 'dart:async';
 import 'dart:isolate';
 
 import 'package:stack_trace/stack_trace.dart';
 
-import 'asset_id.dart';
+import 'asset/asset_id.dart';
 import 'utils.dart';
 
 /// Converts [id] into a serializable map.
 Map serializeId(AssetId id) => {'package': id.package, 'path': id.path};
 
 /// Converts [stream] into a [SendPort] with which another isolate can request
 /// the data from [stream].
 SendPort serializeStream(Stream stream) {
   var receivePort = new ReceivePort();
   receivePort.first.then((sendPort) {
@@ skipping 92 matching lines @@
 /// A regular expression to match the exception prefix that some exceptions'
 /// [Object.toString] values contain.
 final _exceptionPrefix = new RegExp(r'^([A-Z][a-zA-Z]*)?(Exception|Error): ');
 
 /// Get a string description of an exception.
 ///
 /// Many exceptions include the exception class name at the beginning of their
 /// [toString], so we remove that if it exists.
 String _getErrorMessage(error) =>
   error.toString().replaceFirst(_exceptionPrefix, '');