Move a bunch of src/ files into subdirectories.

lib/src/test_case.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 unittest.test_case;
-
-/// This is a stub class used to preserve compatibility with unittest 0.11.*.
-///
-/// It will be removed before the next version is released.
-@deprecated
-abstract class TestCase {
-  int get id;
-  String get description;
-  String get message;
-  String get result;
-  bool get passed;
-  StackTrace get stackTrace;
-  String get currentGroup;
-  DateTime get startTime;
-  Duration get runningTime;
-  bool get enabled;
-  final isComplete = false;
-}