Move a bunch of src/ files into subdirectories.

lib/src/backend/declarer.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.declarer;
+library unittest.backend.declarer;
 
 import 'dart:collection';
 
 import 'group.dart';
 import 'invoker.dart';
 import 'test.dart';
 
 /// A class that manages the state of tests as they're declared.
 ///
 /// This is in charge of tracking the current group, set-up, and tear-down
@@ skipping 69 matching lines @@
   /// groups or at the top level.
   void tearDown(callback()) {
     if (_group.tearDown != null) {
       throw new StateError("tearDown() may not be called multiple times for "
           "the same group.");
     }
 
     _group.tearDown = callback;
   }
 }