Move a bunch of src/ files into subdirectories.

lib/src/backend/invoker.dart

 // Copyright (c) 2015, 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.invoker;
+library unittest.backend.invoker;
 
 import 'dart:async';
 
 import 'package:stack_trace/stack_trace.dart';
 
-import 'expect.dart';
+import '../frontend/expect.dart';
+import '../utils.dart';
 import 'live_test.dart';
 import 'live_test_controller.dart';
 import 'state.dart';
 import 'suite.dart';
 import 'test.dart';
-import 'utils.dart';
 
 /// A test in this isolate.
 class LocalTest implements Test {
   /// The name of the test.
   final String name;
 
   /// The test body.
   final AsyncFunction _body;
 
   /// The callback used to clean up after the test.
@@ skipping 146 matching lines @@
               new State(Status.complete, liveTest.state.result));
 
           // Use [Timer.run] here to avoid starving the DOM or other
           // non-microtask events.
           Timer.run(_controller.completer.complete);
         });
       }, zoneValues: {#unittest.invoker: this}, onError: handleError);
     });
   }
 }