Move a bunch of src/ files into subdirectories.

lib/src/backend/group.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.group;
+library unittest.backend.group;
 
 import 'dart:async';
 
-import 'utils.dart';
+import '../utils.dart';
 
 /// A group contains multiple tests and subgroups.
 ///
 /// A group has a description that is prepended to that of all nested tests and
 /// subgroups. It also has [setUp] and [tearDown] functions which are scoped to
 /// the tests and groups it contains.
 class Group {
   /// The parent group, or `null` if this is the root group.
   final Group parent;
 
@@ skipping 47 matching lines @@
     if (parent != null) {
       return new Future.sync(() {
         if (tearDown != null) return tearDown();
       }).then((_) => parent.runTearDown());
     }
 
     if (tearDown != null) return new Future.sync(tearDown);
     return new Future.value();
   }
 }