Move a bunch of src/ files into subdirectories.

lib/src/util/dart.dart

 // Copyright (c) 2015, 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 unittest.dart;
+library unittest.util.dart;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
 
 import 'package:path/path.dart' as p;
 
 import 'isolate_wrapper.dart';
 import 'remote_exception.dart';
 
@@ skipping 46 matching lines @@
   Isolate.spawnUri(Uri.parse(message['uri']), [], message['message'],
           packageRoot: packageRoot)
       .then((_) => replyTo.send({'type': 'success'}))
       .catchError((error, stackTrace) {
     replyTo.send({
       'type': 'error',
       'error': RemoteException.serialize(error, stackTrace)
     });
   });
 }