Move a bunch of src/ files into subdirectories.

test/utils.dart

Index: test/utils.dart
diff --git a/test/utils.dart b/test/utils.dart
index d8e3e9decd3983a39a0e9cddc53c376b97a5a180..97316e08c9fa836e8b7bb6b04471d77ff1b3ccdf 100644
--- a/test/utils.dart
+++ b/test/utils.dart
@@ -7,12 +7,12 @@ library unittest.test.utils;
 import 'dart:async';
 import 'dart:collection';
 
-import 'package:unittest/src/invoker.dart';
-import 'package:unittest/src/live_test.dart';
-import 'package:unittest/src/load_exception.dart';
-import 'package:unittest/src/remote_exception.dart';
-import 'package:unittest/src/state.dart';
-import 'package:unittest/src/suite.dart';
+import 'package:unittest/src/backend/invoker.dart';
+import 'package:unittest/src/backend/live_test.dart';
+import 'package:unittest/src/backend/state.dart';
+import 'package:unittest/src/backend/suite.dart';
+import 'package:unittest/src/runner/load_exception.dart';
+import 'package:unittest/src/util/remote_exception.dart';
 import 'package:unittest/unittest.dart';
 
 // The last state change detected via [expectStates].