Move a bunch of src/ files into subdirectories.

lib/src/frontend/throws_matcher.dart

Index: lib/src/frontend/throws_matcher.dart
diff --git a/lib/src/throws_matcher.dart b/lib/src/frontend/throws_matcher.dart
similarity index 97%
rename from lib/src/throws_matcher.dart
rename to lib/src/frontend/throws_matcher.dart
index 6807e6d248379ded62e7c6cc6fbef9a04db93ad2..5d672f98a564baa0aa4ddfa19a57edc3c10ff551 100644
--- a/lib/src/throws_matcher.dart
+++ b/lib/src/frontend/throws_matcher.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.throws_matcher;
+library unittest.frontend.throws_matcher;
 
 import 'dart:async';
 
 import 'package:matcher/matcher.dart' hide fail, expect;
 
+import '../backend/invoker.dart';
+import '../utils.dart';
 import 'expect.dart';
-import 'invoker.dart';
-import 'utils.dart';
 
 /// This can be used to match two kinds of objects:
 ///