Move a bunch of src/ files into subdirectories.

lib/src/frontend/prints_matcher.dart

Index: lib/src/frontend/prints_matcher.dart
diff --git a/lib/src/prints_matcher.dart b/lib/src/frontend/prints_matcher.dart
similarity index 98%
rename from lib/src/prints_matcher.dart
rename to lib/src/frontend/prints_matcher.dart
index 4a7c0179f4979e9d98e0942510e835692955760f..55d3fbcaea041ca0a25a72f08b050c3f5ddd2c8e 100644
--- a/lib/src/prints_matcher.dart
+++ b/lib/src/frontend/prints_matcher.dart
@@ -2,14 +2,14 @@
 // 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.prints_matcher;
+library unittest.frontend.prints_matcher;
 
 import 'dart:async';
 
 import 'package:matcher/matcher.dart' hide completes, expect;
 
-import 'future_matchers.dart';
 import 'expect.dart';
+import 'future_matchers.dart';
 
 /// Matches a [Function] that prints text that matches [matcher].
 ///