Move a bunch of src/ files into subdirectories.

test/loader_test.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.
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
-import 'package:unittest/src/loader.dart';
-import 'package:unittest/src/state.dart';
+import 'package:unittest/src/backend/state.dart';
+import 'package:unittest/src/runner/loader.dart';
 import 'package:unittest/unittest.dart';
 
 import 'io.dart';
 import 'utils.dart';
 
 Loader _loader;
 String _sandbox;
 
 final _tests = """
 import 'dart:async';
@@ skipping 108 matching lines @@
 
       test("can run tests in those suites", () {
         var suite = suites.firstWhere((suite) => suite.name.contains("a_test"));
         var liveTest = suite.tests[1].load(suite);
         expectSingleFailure(liveTest);
         return liveTest.run().whenComplete(() => liveTest.close());
       });
     });
   });
 }