Move a bunch of src/ files into subdirectories.

lib/src/frontend/future_matchers.dart

Index: lib/src/frontend/future_matchers.dart
diff --git a/lib/src/future_matchers.dart b/lib/src/frontend/future_matchers.dart
similarity index 96%
rename from lib/src/future_matchers.dart
rename to lib/src/frontend/future_matchers.dart
index 2ae13eebd367df34276a7e6ea6b5a10718d21c84..98569ded8071fc7b7068fdb8fc1a56fcd4afbc6a 100644
--- a/lib/src/future_matchers.dart
+++ b/lib/src/frontend/future_matchers.dart
@@ -2,15 +2,15 @@
 // 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.future_matchers;
+library unittest.frontend.future_matchers;
 
 import 'dart:async';
 
 import 'package:matcher/matcher.dart' hide throws, throwsA, expect, fail;
 
+import '../backend/invoker.dart';
+import '../utils.dart';
 import 'expect.dart';
-import 'invoker.dart';
-import 'utils.dart';
 
 /// Matches a [Future] that completes successfully with a value.
 ///