Move a bunch of src/ files into subdirectories.

lib/src/frontend/future_matchers.dart

 // Copyright (c) 2012, 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.
 
-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.
 ///
 /// Note that this creates an asynchronous expectation. The call to `expect()`
 /// that includes this will return immediately and execution will continue.
 /// Later, when the future completes, the actual expectation will run.
 ///
 /// To test that a Future completes with an exception, you can use [throws] and
 /// [throwsA].
 final Matcher completes = const _Completes(null, '');
@@ skipping 48 matching lines @@
 
   Description describe(Description description) {
     if (_matcher == null) {
       description.add('completes successfully');
     } else {
       description.add('completes to a value that ').addDescriptionOf(_matcher);
     }
     return description;
   }
 }